I prefer to balance on a balancer, even when using electrics. Then again, when you are in the field.... there are other ways. Here's a nice video with some quick tips (they only mention it briefly, but they do mention how to balance a mounted prop on a plane. Put tape on one side, if it vibrates worse, you put the tape on the wrong side!):
http://goo.gl/fFCwmRegards
Christian