||It is very hard to know from your interview. To quote one of the instances, we did interview a few people but they were supposed to join other department. If the interviewee concluded something based on the way we spoke to him, it would be an inaccurate conclusion.
If you want to know whether they are technically good, you can do to a certain extent but if you want to know whether they will be good mentors, I doubt if one can conclude without working with them.
Its always a better idea to speak to an ex-employee of the company to know about a dept and the people in the department.