Suppose a candidate claims knowledge and experience in a certain field on his resume that is not relevant for the job to which he is applying, and that one of the interviewers also has knowledge of that field. Is it appropriate to ask questions of the candidate to verify that knowledge even though it is not needed for the job? The purpose would be only to check whether the resume contains exaggerations or false claims.
Is it appropriate to ask questions of the candidate to verify that knowledge
Yes it is, you can ask any questions you like to gauge a candidates honesty. If they claim competency then they should have no issue with it. I wouldn't spend much time on it or make it a focus but I've often asked unrelated questions in interviews for similar reasons.
It's OK, but it's not brilliant because it means you're applying a different standard of scrutiny to candidate A (who shares your irrelevant interest in rock-climbing and therefore you can really quiz them on their claims about it) from what you're applying to candidate B (who claims to be really into sailing, but you can't test whether they're telling the truth or not since nobody on the panel can sail). Under what possible circumstances would you be content to let B get away with their lies but not A?
Ideally you can establish whether candidates are on-the-whole honest by quizzing them on the job-relevant claims in their CV. If you can only test their honesty by quizzing them on irrelevant stuff from their CV, then you've pretty much already lost. Which doesn't mean you can't do it, but it does mean you should try to arrange your affairs so that you don't need to do it. If you really need to test every claim on A's CV, then surely you need to test every claim on B's CV, so that means serious research of whatever they claim, not just testing whatever the interviewers happen to know about. If you're resigned to not being able to test B's irrelevant claims because you don't have the knowledge, then why waste time testing A's instead of talking about relevant things?
If the questions had some other purpose than testing honesty (for example establishing rapport, putting the candidate at their ease, giving the candidate an opportunity to speak about something they know about but most of the panel doesn't to test their ability to communicate with ignorami), then you'd quite possibly be on to something in asking questions about irrelevant stuff on their CV. Since they don't have any other purpose, I think you need a more reliable means of testing honesty that can be applied to every candidate, and that means testing the relevant claims.
Mind you, if a CV contains a claim that's obviously dubious, and you happen to have the knowledge available to test it, then it's probably not a waste of time to apply additional scrutiny to the candidate with the obviously-dubious claim.
It might depend on the type of experience and how uneasy you feel about the candidate.
For instance if the candidate was a stretch for the position based on his experience in my specific stack but had extensive experience in something relatively similar, I would want to verify that he knew what he was doing in the other experience.
So the the guy with 6 months of C# and 3 years of Java might get some Java questions, same with say SQL Server and Oracle. This is particularly true if the position was higher than entry level. This is especially true if I wasn't completely satisfied with the experience level shown when asking about the stack I do want him to use because I want to determine if he is simply not good at anything, lying about his experience or just new to the new stack and not as knowledgeable yet.
Someone who fluffs some C# questions who is rock solid on Java is someone who can be trained to do C#. Someone with years of experience in Java who can't answer those questions or the C# questions, I would probably pass on.
But would I ask the guy about his experience with Hadoop when it is not even closely related to what I do and when he has plenty of verifiable experience in C#. Probably not. I might ask some generalities around that type of experience just to know what kinds of things he did and get a view for his thought process. But not specific technical questions. With that kind of experience I am more interested in find out more about how much he accomplished and what his soft skills were like than whether he was technically proficient.
But if he was having trouble with the questions about the stack we do use and had some completely unrelated experience I would probably pass because programming embedded systems doesn't give me any better feel for whether he could do a good job in database work than accounting experience might.
You are recruiting someone, you are the one choosing your recruting strategy. If checking if your Python programmer for your accounting firm REALLY knows about teaching soccer to kids as he says is part of your recruiting strategy, it's your call.
Said otherwise : what is important, in your eyes? Compliance to the job? Or being a well-balanced human-being? The answer is not obvious, and probably one of the most interesting choices when recruiting.
There is another side-effect : if the candidate did not lie, it can create a bond between him & your other worker who knows about the topic. Good for the team.
Well you've given away the answer in the comments to your question. It is not the answer to the question in your title but sounds to be the answer to your actual situation.
Technologies a technologist knows but do not happen to be used in the current position are extremely relevant.
Think: are they smart overall? Can they build stuff without it breaking? Do they have engineering know-how? Do they understand that code needs to be "good" or does it just sort of have to work? Do they get confused when they have to solve a hard problem? Do they pout when the requirements are vague? How do they test their technology? Have they built something that a life depends on? Or millions of dollars?
Biotechnology or web development, these are core questions to being an industrial technologist.
To add some personal experience: out of college, with an electrical engineering degree, I went to an interview (for what would become my first post-degree job) for a software development position (my focus in electrical engineering had been in programming, and I also got a minor in computer science).
One of the interviewers was apparently a car enthusiast, and decided to throw out a random question about his car’s wiring. I was a bit flustered; my degree covered a fair amount of circuit design, of course, but nothing car-specific, and I really didn’t (and don’t) have significant experience with cars beyond driving them. My classes had been very academic, theoretical, and he was asking a very practical question.
Luckily, the other interviewers teased him about “trying to get me to fix his car” and we laughed about it and moved on, but it wasn’t exactly a great moment for me. For that moment before the other interviewers turned it back on him, I was feeling like I might need to back up my resume’s claim of electrical engineering expertise, which I wasn’t really prepared to do: for the past year or so, I’d been focusing almost entirely on the software side of things, and that’s what I’d come to the interview prepared to demonstrate.
So I would say that, if you take this route, be prepared for the possibility that someone’s interest in a particular field may not overlap as much with yours as you think, and he or she may not actually know the answer to your question – without lying. It can also catch a person off-guard, and being an unexpected question that they haven’t prepared for in the least (not even related material), their being flustered may very well be nothing but that.
This answer is going to be a bit of a hair splitting exercise. You should encourage the interviewer to ascertain the character of the candidate. You should not encourage the interviewer to verify non-essential knowledge from their resume. The two are different not in what is verified, but how and why the verification is taking place. That makes all the difference.
If you are simply testing to see if they padded their resume, that is nothing more than that: a challenge to their character. In a perfect world, you will only have perfect candidates who are not offended at a "innocent" jab at their character. In the real world, you may offend a potential employee.
However, if the interviewer has reason to test the credibility of the candidate, then they should feel free to use whatever tools they find best for the task. Maybe the candidate is a little shaky on something he was supposed to know. Questioning a non-essential aspect of his resume may help show whether this is just a person who got nervous at an interview, or someone who is inflating themselves on paper.
I would definitely make sure the interviewer knows such questions are fair game, not from a "test their character" perspective, but because you have a unique chance to get closer to the candidate than you would otherwise get. The interviewer and candidate share a common field that is external to the work. This could be incredibly useful for an interviewer to help see the person they are interviewing, not just the skillset for hire.
This case builds the best of both worlds. Not only do they get to build a bond, but they will get to verify the knowledge of the candidate as a side effect. There doesn't have to be a sense of putting the candidate on trial, just a honest candid discussion between two people about a common interest. If the candidate is authentic, you may actually have an opportunity to make them more interested. If they are not authentic, then you didn't want them as an employee anyway, and the interview can turn from friendly conversation to test quite rapidly.