How to set the gain on XM-GTX1852 1000 Watt Sony Amp?

Like Where do i set it i got it pushing two 07C124 12" Kicker Comps. and i got it about 3/4 of the way gain up. but im not real sure on how to tell if the subs are clipping or distorted? subs specs are 4ohms SVC 150 RMS 300 PEAK. Amp Specs. 185 x2 @ 4ohms. if anyone knows how high i should set it please let me know. like i said i dont know how to tell if subs are clipping or distorted, im guessing it would be pretty noticeable if anyone could explain