I am beginning a doctoral program this Fall and have been receiving helpful advice from a recent graduate. While this graduate has mentioned a key mentor that will likely serve as my primary advisor, I am curious to know if there were other mentors instrumental to his success in the program.

What might be a delicate way of inquiring who these people are?

I am beginning a doctoral program this Fall and have been receiving helpful advice from a recent graduate. While this graduate has mentioned a key mentor that will likely serve as my primary advisor, I am curious to know if there were other mentors instrumental to his success in the program.

What might be a delicate way of inquiring who these people are?

EDIT: I was initially afraid that having someone identify the "good guys" might imply that I reckon that some are "bad." But this is probably overthinking it, as one comment pointed out.