Sometimes you won't hear anything.
Let's say you had a subwoofer in a folded horn, and you were running it at it's continuous top rated power. Folded horns by nature lower distortion and other artifacts. The driver can get hot and melt and you don't know until it's too late.
Another extreme is someone using a lot of distortion. If you run the driver past it ratings, and it's good tuned box, you won't always hear the driver stressing until you blow it.
FOH uses Limiters. Most DSPs allow for "Watts" or "DB" limiting.
Eminence has new type of limiter http://www.eminence.com/d-fend/
It's actually a switching amp in reverse. It's interesting but just limiting can solve a lot of problems.