well, what model are you using?
Different models have different abilities. Some wont distort, others will...
to be sure, your going to have to get an oscilloscope and some specialised test tracks. Or wait for advice off here, or follow the ghetto method.
You look for clipping on the oscilloscope's display, while turning the volume up.
Now, for the ghetto method

turn your amp gains down to minimum, then start turning your headunit up. If the music starts distorting, back off a little. Thats your maximum headunit volume. If the music doesnt distort, (goes all the way up to max volume without being harsh) your headunit doesnt clip/distort.
IF it sounds like your hu is distorting, find the maximum volume before distortion occurs.
Leave it there.
Then start gradually turning your amp's gain up untill distortion is heard, then back off a little. You'll now have your system tuned for maximum loudness. Take note of your headunits "maximum volume before clipping", and NEVER turn it above that level.
You should do this once with your sub disconnected (adjusting the front speakers gains only) and do this once with your front stage disconnected (adjusting sub only). You should also have ALL eq/bass boost/loudness functions turned off while doing this.