Sight down the neck and look for a wavy pattern on the frets. If the neck is straight, you should be able to see the tops of all the frets evenly. If it's warped, the fret ends will look to be higher on some spots of the neck and lower on other spots.
Don't confuse a warped neck with one that has some excessive relief or back bow, as the fret tops will look kind of similar but not the same (they'll look as if they're closer together depending on the relief or backbow won't look warped). Relief and back-bow can be fixed with truss-rod adjustments, while a warped neck requires more drastic treatment... possibly a fret job with fingerboard planing, or even a neck replacement depending if the neck continues to warp after getting work done.
Some in the guitar repair community tried working on a system that would "twist" the neck back into spec using heat and various clamps, but -at least as far as I can remember- results were kind of questionable. That might possibly be another avenue to look into.
