Just turning the gain all the way up is generally not a good idea, unless you have really poor signal voltage to the amp. Use your ears, and sometimes your nose. If you can smell your sub, that's a sign you're getting careless. If you turn the volume up and up then the sound starts to change, that's a sign of distortion(not necessarily clipping). When we are new at this, it is harder to detect distortion.
Best way I can think of to try and hear the difference would be as follows. Listen to a familiar song multiple times at a moderate volume level with everything balanced out(not bass heavy, etc.). Just loud enough so you get the full spectrum of the song. Then, once you feel you can listen to the song and predict the little nuances and details start turning it up slowly. Once you start to hear things change(lose small details, tones change, lose clarity) it's time to back off a tad. If you start getting too crazy you end up hearing screeching, buzzing, rattling, etc. from the speakers themselves when you hit their mechanical limits and break things for sure.