Check for wing twist. This is easy on a flat-bottomed trainer wing by rubber-banding a dowel (or arrow, fish-rod, etc) to the same point on each wing panel and looking at them from the side to see if they are even. Check at the root and again at the tips.

If the trim isn't enough you have to adjust the linkage (which you should do anyway rather than rely on trim or sub-trim settings).

Balance the model laterally by supporting the prop-shaft and a rudder hinge with thread. If one wing always drops add weight on the end of the opposite wing. This shouldn't be enough to upset a trainer unless it's really bad.