If you push your amp into clipping it will have a progressively deteriorating sound quality. It will start by just sounding 'uncomfortably loud'. After a short while you will have the overwhelming feeling that you need to turn the volume down.
If the symptoms of a clipping amplifier are uncomfortably loud sound then I'm sure one could use the same symptoms and misdiagnose the problem for something else entirely. Like say the room acoustics. Perhaps the reflections in the room at a given volume are so harsh that it sounds too loud at that setting, so instead of addressing the acoustics in the room, they decide to buy a bigger amplifier.
I guess what I'm trying to say is that you could probably have more than one symptom that may also reach similar conclusions - re uncomfortably loud. I'm not a technical guy, but I imagine you would probably need switching gear that could toggle between two amplifiers, one a lesser amp and a more powerful model otherwise it may just be my brain playing tricks on me. Either that or some way of measuring the signal at that volume, as arny described.
I don't know, what do you think? Thanks again for your explanations in the thread, I'm glad we could all have a civil discussion about this.
There are several possible causes of distorted sound - Arny has listed them in his post above. I was just responding to the poster's question where he asked for a simple description of what clipping sounds like. An experienced listener will probably be able to distinguish between the sound of an amp clipping and the sound of an overly-reflective room causing distortion, but for an inexperienced listener I can easily believe they might sound the same. But remember that 'sounding loud' was only one of the possible causes I gave. Arny has explained how to test for sure in his last post above. It is bound to require some technical ability to carry out such a test of course.
Again, the bottom line really is that if your system sounds good at the SPLs you want, then you are basically all set.
EDIT: if you have a computer that can play sound files, why not take Arny up on his offer of providing some safe downloadable files that demonstrate what clipping sounds like.