without on oscilliscope..

turn your head unit to ~75 percent up
turn the gain on your 4 channel all the wya way down, put it in a cd.
slowly turn the gain up till the speakers distort, back it down just a bit
turn off the car
unplug the rcas to the 4 channel
turn the car back on, and repeat withthe sub amp
turn volume down, and plug 4 channel rcas back in

only reason I saw to not have the signal to the 4 ch is just to save any outside noises from interfering with the sub gains.