Are you talking like an unusual attitude recovery class, basic acro intro class, or teaching competition aerobatics?
At the very low end 100 hours of recent tail wheel experience is often an entry point the insurance company will require before you get coverage to instruct in a taildragger, which most acro airplanes are. If anything beefier than a Citabria, they'll likely require significant time in type too.
Then for teaching the basics, I'd look for someone who's been through some professional acro training from a credible school (professional competitors, IAC affiliation, Stowells EMT/acro, etc). Obviously before teaching competition the person should have several seasons of competition experience and a proven track record showing they know what they are doing. The more accomplished they are in the acro world the more I'd trust they know what they are doing, but there's also the personality issues of "can they teach"...so some good references might help with that. Holding an ACE card (low altitude waiver) would indicate they've proven their judgement and abilities recently before other accomplished acro pilots and should be pretty meaningful to the prospective student.
I mean pretty much any joker can hack their way through a sloppy loop, aileron roll, and spin demo and the ignorant passenger/student will think it's awesome fun... but can they explain what's really going on and can they help teach you through the "when things dont go as expected" issues?