Straight neck?

[brymenez94]

Is there a way to check if your neck is completely straight? (Other than obviously looking at it, of course.) I think that my neck might be already near being dead straight, but I want to be sure.

[rojo412]

Strings always pull dead straight, so eyeball it against them.

Or get a straight-edge, like a 24" ruler and see if there is a gap under the middle when it rests on your first and last fret, strings on, of course.
Or if it teeter-totters on the neck, as well.

[Jaco D]

Get a capo and put it on the first fret. If you have the bass slung on your shoulder, use your right arm to push the strings against the bass body so that the strings are flush against the fret tops. Assuming there's no wear on your frets and they've been leveled properly, there shouldn't be a gap between the strings and the frets from the first to the last fret.

[Muaguana]

Or just fret one string at the first fret with your left hand and the last fret with your right, and judge the size of the resulting gap between the string and frets. You'll see the curvature of the fretboard, if any.