Tie a loop in the middle of a piece of string so you have two equal lengths of string on either side of the loop. Now tie a loop on one end to loop over the propellor. Secure the other end of the string to the rear of the plane (if the plane has flying wires, tie to them. Otherwise you'll have to customize the fastening technique.) Make sure the lengths are still close to equal (it doesn't have to exact). Now hang the plane from the ceiling, making sure the string is as far forward (behind the prop) as you can. It can be a bit un-nerving to hang your plane this way but it works.
Get a plumb-bob and hang it from the same hook the plane is hung from. Let everything come to rest and the plumb-bob will be pointing to the C.G. of the plane. I use an empty film canister and BB's to then balance the plane.

This method will allow you to find the C.G. and the lateral balance. I use it when balancing my Somethin' Extra.