How to detect a neck warp?

[imc2111]

What's the most practical way to detect a neck warp? I know some can do this just by eyeballing the fretboard.

[Bill Lowther]

Straightedge?

[Rodger Knox]

Play the guitar! If it doesn't have strings on it, looking down the neck should work. Just to be sure we're talking about the same thing, a warp is more(or less) curvature on one side of the neck. That's different from bow or relief.

Obviously, a very small warp may not be visible, but a warp that small usually isn't a problem.

[arie]

initially, sighting down the neck works for me. when held right, the frets will align themselves like a vernier scale and one can spot trouble pretty quick. if i do spot anything out of the ordinary, then it's time to get out some measuring stuff and investigate further.

imo, sighting down the neck is good for a first pass.

[charles Tauber]

You'll need to better define what you mean by "neck warp". Twist? Bow? Humps and valleys?