Rear sub frame is easliy bent. I've seen a few of them this way.
Do check the swingarm bolts, the adjustable one should be on the right.
Check you spacers and stuff. There shouldn't be one inside the drive hub spline area. the right side, from the wheel out, should have the small spacer, the brake mount, then a washer.
I kind of like the idea of a plumb bob from the steering neck, then a straight line from there going forward and back, if the straight line intersects the front wheel center while the wheel is pointed straight (assuming your forks aren't bent) then extending it back should give you your true off set I think.....
I'm confused as to how you could have an off set to the left unless either the frame or swing arm is bent or else you have adjustable swingarm bolts on both sides of the swingarm???:confused2: