I don't think there's any way to do it "100 percent" from detecting an audio signal on the RCA cables. It might "false" once in a while due to noise or such. Although I know it's do-able because I've got a subwoofer at home which automatically powers on when it gets an audio signal. But mostly I'm not worried about that "falsing" because there's no drawback to having it switch on unexpectedly, other than a little power gets wasted. With the empeg, on the other hand, it would be very irritating to be listening to MP3s and have it "false" and switch to the Aux line in the middle of a song.

Most devices that you'd want to do this would have some other way of indicating power-on status, something more reliable to test against. And probably simpler, so that the most you'd need is a relay.

How about you tell us what device you intend to do this with and we can think of a simpler solution?