It's just nasty distortion, that's extreme clipping. It's very hard to pick up minor clipping, but transistor amps keep their distortion low until clipping, and when they start clipping, they clip HARD. You'll notice a huge increase in distortion, you'll also notice the music sounds a lot louder and more harsh, especially in really high frequencies, tweeters will absolutely scream when they are clipping. Only way to truly know is to hook an amp up to an oscilloscope.