I can usually eyeball most warps, but an easy way is to lay it on a flat table and see if there is any deviation from the root to the tips. Unless there is intentional washout built in, each side should lay flat on the table. It doesn't take much to cause problems - a 1/16" deviation from root to tip will show up in the air. The problem with "trimming out" things like minor warps is the trim you need really varies with the speed of the plane - better to remove the warps and shoot for 0/0/0 on the trims.

Using trim to compensate for warps also causes more drag and causes non balanced throws. Not such a big deal on a slow stable Nexstar, but on something that's got a symmetric wing, no dihedral, and a roll axis near the wing C/L, this can cause the plane to not track straight through a loop and do rolls that are wobbly around its roll axis. I see all kinds of planes out at the field exhibiting these "trimmed out warp" symptoms.