What exactly is clipping?
For subs you can't hear it unless it's REALLY bad. If your subs are setup in a ported box, or just some way where the actual "internals" of the sub are exposed somehow you will smell it if it starts to heat up a lot. Alternatively, if you play music full tilt for a couple minutes and feel the dustcap and it's very warm or hot, chances are you may be clipping. Keep in mind that sending a lot of power to your sub will do this as well, so sometimes it's hard to tell when it's clipping or notMakes sense, can you hear the clipping? How do you know if it's doing it?