Buy a multimeter. Use the continuity test and check the each of the 2 points between two of the buttons. If 2 points on the different buttons trigger the beep (or whatever signal the multimeter uses) those are more then likely ground points. Then check all the buttons against that ground to see if it is common ground or not. Win.
No insult intended but anyone who is trying to mod with unknown parts should own a multimeter. Just go to your local electronics store or amazon for a digital multimeter. It should have a little speaker icon or diode symbol for the continuity test (mine has a speaker).
Right, I meant it more for general advice on how to map out unknown or difficult PCBs. You’ll probably have to plug it into the system and poke around to see what’s what, if it’s too hard to tell by looking at the PCB itself. Or use a continuity tester like Kakitajamie mentioned. Though you don’t need a whole multimeter necessarily, you can get really simple continuity testers that just have a light or buzzer and a battery.