use a multimeter in voltage test mode put the negative on a ground, i think those big gold plates are im not sure and put the positive on each one until you get a reading, anywhere between 1.5-5 volts will do
the easier test but far less rudimentary test that i sometimes use is to just solder the negative of your led to a ground, then insert the guitar hero attachment into to the ds, power it on and touch the positive of the led to each of those points.
but that doesn't look a whole lot like its the place to be soldering too..... if you know which pins in the phat gba slot give out a voltage (it'd be the same as the ds lite id imagine) just find a trace that comes off that pin and solder away, but if you sure if thats the place then try the above.
