if you have ailerons are they faired with the wings when aileron trim is neutral and the stick at neutral? if you don't have ailerons how about the rudder? do both ailerons or the rudder move with stick input and trim? How much power are you using when flying? If your thrust line is off it'll pull the aircraft. so if it pulls more with throttle up than it does with throttle retarted I'd start with engine thrust line.
Oh and by the way is the wing centered on the fuse and have the same amount of curve on both sides?
Any way, start there and let us know.