The first step to determining whether a guitar’s neck needs to be adjusted is to check its relief, or the amount of bow the neck has. Here’s a tip for making this easier: you can use a straight edge to help determine the neck’s straightness (or lack thereof). But your guitar actually has a built-in straight edge – the strings. To determine neck relief, simply tune the guitar to pitch, then fret the low E string at the first fret with one hand, then at the last fret with the other. The string will form a straight line, allowing you to use a feeler gauge to measure the amount of relief at the 6th or 7th fret.
Bonus tip: to make it easier to check relief, use a capo at the first fret. This frees up both hands; one to fret the string at the last fret, the other to do the measuring.