An old carpenters' trick is to use winding strips, i.e. two straight thin battens of wood across the neck and at right angles to it. Then sight down it and any 'winding' (warpage) is obvious. Of course, this only works with a non-radiused f/b; for a radius I would place a strip in the nut slot and sight on to the body.
Also, provided the guitar has been built correctly and your headstock is symmetrical it is possible to sight from the headstock on to the heel and spot any twist.
I've had quite a few guitars in for set up which have had greater relief on the bass side, as much as 10 thou difference, and as none of these have been particularly well known makes I'm in the dark as to whether it was intentional, poor workmanship, or a distorted neck. Still, I'd rather have this, than too much treble relief!