Place a scale under one wing tip, and block up the the other wing in the same postion on it's tip. The plane is then suspended by both tips. Note the weight on the scale. Now, reverse the scale and the blocking to the other tips. Note the weight again. You now have your light wing tip, and by how much. Just add the difference to the light wing, or better yet, move your batteries in the fuse toward the lighter side.

Hope that helps.......