If the wing is standard balsa construction with a MonoKote type covering, it is very easy to remove warps. Simply twist the wing in the other direction until the covering wrinkles badly, hold that twist and run a heat gun over the covering to pull the wrinkles out. Check for twist now. It may require several applications but reshrinking the covering will provide enough tension to hold the twist in the wing.

Of course the best way to determine the twist is with an incidence meter, test incidence at the root vs. the wing tip, then twist and heat as needed. As CK1 said, a bit of washout can be a good thing on a trainer, but be sure you have the same amount on both wings.

Dave