It's too bad the wiring diagram doesn't show a schematic of the inside of the J538 like it does some relays so you would know what it does. One or both of the ECU wires are supposed to be a signal telling the relay to go from 10V to 14V for the fuel pumps. I can't imagine an easy way to find out. You could put a jumper on both wires and go through a start/drive procedure and see what kind of signal/voltage the put out and when but it wouldn't be 100% helpful, maybe. If you saw a signal out of one of the wires as soon as you gave full throttle then you would know that is the one that kicks the fuel pump power straight to Alternator power. Sorry, I'm not much help without a better schematic, looks like you've come as far as I can already.