if you would like to know exactly when your amp is clipping, without having to guess, get amps that have LED clipping lights; for me, it's the surest way i know of.
another way is to hookup your amp output with an oscilloscope in parallel with one of the front speakers. then you can see very clearly when clipping happens.

See: http://www.blu-ray.com/community/gallery.php?member=Gelli
or: Axiom Gallery