A good mentor points out the spinach in your teeth before the interview, not after. Find one.

What makes someone useful to you is not how much they know but whether they will say the awkward thing while it can still be fixed. Plenty of people will tell you afterwards. The rare ones tell you at the door, when it is small and correctable and slightly uncomfortable for them.

Look for someone a few steps ahead who has already been honest with you about something minor. Ask them to look at the thing before it goes out, not after. Make it easy to be blunt with you, and they usually will be.