Ryan,
I use a continuity tester. This is a function on my multi-meter. Touch one end of the tester to ground and then poke around the board to see what else is connected to ground. 
If you don't have a multi meter and you do electronics work with any amount of regularity (more than once every few months) you should really get one. It doesn't have to be fancy, but you should make sure it has a continuity tester on it. Some cheaper ones don't. Expect to spend $20-$50 for a "good enough" model.