Estivation can occur when it's too hot. Hedgie's activity decreases/becomes lethargic - so, in that way, it can look somewhat similar to hibernation. However, you'll probably notice a "splat" that indicates hedgie is too warm. They look like cute little starfish, all stretched out, trying to cool off - so in that way, it looks very different from hibernation where they're curled into a ball.
I haven't really seen what happens after splatting out. Nor at what temperature estivation will officially set in. I know my quillkids have been in mid-80s without issues. Others have reported their quillkids being okay at upper 80s.
I'd start officially worrying around upper 80's, but that's my gut feeling talking; not formal research.
My understanding from some resources are that estivation is as bad as hibernation attempts; others have mentioned it's bad, but not quite as bad as hibernation attempts.