One way to check is to do an inside loop from upright flight and then an outside loop from inverted flight. See which wing tends to roll to the outside. If it's the same wing panel (say, the model's right wing panel), then you have a weight problem. If its one wing panel in the inside loop, and the other wing panel in the outside loop, then you have a misalignment somewhere (wing warp, ailerons not aligned, bent fuse, rudder off, elevators not aligned, and so on).