I've revised this answer.
The way how to look at colleges, is both off-line and on-line search.
On-line search reveals websites that sell certificates
When doing an on-line search, searching the name of the college, you would get websites that sell certificates in the name of that school, or direct ability to buy certificates in courses offered for that school, without having to attend college or tuition.
A dead giveaway is that college or university is a degree mill is number of google search results that give bad publicity to the school and law-enforcement done to shut-down them down.
University of Northern Virginia, Va. university chancellor resigns over ‘sex dungeon’ flap
Checking campus location
You can check Google Maps for the college address. You should see visually, a campus, various low-rise buildings, bus routes and highway exit routes leading to a campus or campuses.
A website which claims to be a college, would not have a huge campus, would keep their phone numbers and off-line address intentionally hidden or obscure.
Checking college website
If you are browsing the college's website, you would see course information posted publicly, how to enrol at that college, syllabus guide what you will learn, telephone number to contact admissions and student's websites, if available.
Is the syllabus guide detailed enough to give employers information about what the student studied during the time the student was there?
Is there a contact number an employer can contact admissions to check if that student enrolled or attended the courses?
A college degree mill, will focus on the certificate selling part, where you can order certificates of course completion along with attendance certificates. That would tell you that is unethical and put the certificates they sell at doubt.
The college website will have links or a page for which bodies they are accredit to, and how to transfer credits out or into their college.
For example, you studied at that college for a year, but wish to transfer out to another college to continue.
A college degree mill will offer no such ability to transfer or receive credits.
You can do social engineering - pretend to be a student who only wants to get a certificate without having to attend classes or pass exams.
Federal laws require that in order for a student aid, the college must be accredited or approved. If you inquire about federal aid, is that the college does have any provisions for federal financial aid. All fees must be paid upfront, or in full prior to attendance.
Colleges keeps student attendance records, student enrolment details, grades earned, coursework awarded in their computer systems.
A college degree mill, will allow students to falsify the date of attendances to anything they fancy, back-date it, or change it, as to reflect full attendance.
Social questions to holders of diploma mill certificates
Since the person asking the question is recruiting, you can the below questions as interview questions -
What is the name of the college professors who taught you, during that time?
Who were your 3 best friends you made during that time? Can I see their LinkedIn profile? (They should have attended the same college during that time)
What did you do during the college semester holidays?
Can you describe your final year thesis? What could you have done better?
Do you have a link to your college website and course syllabus for the Bachelor's degree so I can see what you studied?
Questions regarding profession
You can ask them about the line of work for which they have studied in.
You can often see incompetence or inexperience of the person, poor skill-sets or inexperience in the line of their profession.
a) Bachelor of Information Science, but has no knowledge of system analysis, inexperience with simple programming, not skilled in basic skills, unable to code or write PHP or C# or Java.
b) Bachelor in Human Resources, but have no knowledge of management, how to conduct interviews, how to do basic payroll, how to do assess others
c) Bachelor in Marketing, but have no knowledge about marketing, poor or hostile in meeting customers, unable to perform to expectations.
You can do direct verification. For example, you might ask for consent to check their university's attendance or graduation records, other than photocopies of certificates they give to you.
Usually, no such person exists, or the person never graduated.
I've hired people who buy certificates from a degree mills. The above represents my experience with dealing with such people.