If you've got it all torn down, you can use a long wooden or metal dowel (make sure THAT's straight) through the steering head, then sight from the front, standing back several feet, and if the next is tweaked you should be able to see it.
Another thing is to hang a laser lever from the ceiling, pointing down, and then align the frame under it on blocks so the laser paints a line down the centerline of the frame. Any sideways bend in the backbone will be readily apparent.
mystic_1