How about my supervisors' and HMs' bosses/recruiters/etc contact ME to ask how well they handled their leadership. If I matter so little to them my opinion won't exactly hurt their feelings, now will it? Stop being afraid of me, I'm 5'2″.
TL;DR: Stop talking to the people employers work/ed for and start talking to the people they were supposed to be working WITH. And/or managing or whatever. Make that shit mandatory.