Signal sensing from Tone Board GPIO

Hi, I want to use an arduino to sense when my Tone Board is outputting a signal (data stream, clock signal, analog output etc) so that I can use it so switch a MOSFET. Can anyone suggest an easy way to get this from the GPIO header? I wanted to avoid taking it from the RCA plugs if possible and digital might be easier to use as a trigger in case the tone board is not totally quiet with analog output.

I need to know what is an easy starting point so I can work on best way to get the arduino to reliably recognise when a song is playing.