The sub cools itself by moving air through the gap of the motor. Clipped waves cause brief periods of time where the sub isn't moving, but is still receiving power. What this means is there is a brief period of time where the sub is not cooling itself. Consequently, the coil heats up quicker, and that's when you start running into stuff like smelling the glue from the voice coil heating up/melting
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 not