I checked the lateral balance by taking the prop of and resting the plane upside down of the engine shaft. Then by lifting the plane by one figer from most aft part of the plane (upper tip of the fin) I could easily see wich side of the plane that is the most heavy one. I added a small weight on the lightest side wingtip so the plane got a lateral balance. When doing this one need to check several times by lifting the tail and the wing tip that falls the most times is the heaviest.

Also take off the main wing, hold it upside down with one tip on the floor and and look carefully along its lenght to see if the wing is warped or not. If warped, you can carefully twist the wing back, using your hands, and warm the covering with a heat gun, take the heat away and continue to hold the wing untill the covering has cooled and locked the wing in its new position (hopefully straight as an arrow).

Good luck.