To evaluate a resume writing service, contact people that provide these services and ask them how you would benefit from what they have to offer - get success rates, examples of previous work performed, and references. Find out what kind of guarantee they offer. Also research online and read any articles or comments people have posted about the services that you are considering.
To identify when a resume writing service would be beneficial, you will have to consider what these services have to offer and whether it's worthwhile for you.
For example, you specifically said that you're looking for someone with a track record of increasing the impact of software development resumes, so you ask people to show you their track record of success and then decide if this is something worth paying them to do for you.
Additional information... I don't know if it's appropriate to share off-topic information, but I'd really like to help you.
You might be better of working with head hunters who know how to promote your skills and abilities to prospective employers. This is a very common way to get jobs in software development. And the head hunters might be able to give you some good feedback on how to improve your resume, especially since they get paid when you are successful.
Also, try contacting companies that hire software developers and ask for feedback on your resume. You might be able to reach a hiring manager who is more than happy to help. Plus, you might discover new job opportunities in the process.