I've never seen a reference of it anywhere- have never heard a bootleg - in the Cavett think Jagger shows up during Stevie's set so clearly no soundcheck that day - just curious--
They probably soundchecked and then left the arena for various pre-show orgies at the hotel.
Obviously soundchecks been going on since the first amplified music. Every musician must soundcheck, I did it while playing saxophone in a brass band when I was a teenager, just to check that the reeds were working properly.
There are well documented dress rehearsals from 1989 in Philadelphia. I guess they combined it in the later days. In the 70s maybe the roadys did the check. So Keith could sleep for some more hours ;-)