Here is a way to get you pretty close.

Suspend your bow by the limbs (I use a bow vice). String would be parellel to the ground. Back of bow facing the ground. Put one small level in the corner of each limb pocket. Make sure the bow is hanging level. Hang an arrow with the heaviest tip that you have on the string. I use a 220 grain. Adjust the rest (left/right) to the arrow.

Not the most scientific, but it'll get you real close.

I had one of those devises that Rob listed, however, it never worked well for me. I had a real consistancy problem with it.