How to Check if the Neck is Straight
When setting up a guitar, the neck adjustment always comes first. The neck is one of the main parts affected by changes in the weather and needs to be maintained and adjusted regularly. Here's an easy way to check the straightness of your guitar neck:
First set the tail end of the guitar on your toe and look down the neck towards the bridge. Look under the low E string across the top of the frets. Comparing the line of the neck to the line of the E string, try to see if the neck is as straight as the string or if the neck looks bowed away from or towards the string.
Now do the same under the high e string, trying to determine if the neck is bowed away from or towards the straight line of the string.
Next, sit down with the guitar in playing position and play the F note at the first fret on the low E string. With your right hand press the string down just past the highest fret with your thumb. You want to look at about the 10th fret to see how much space is between the fret and the string. Imagine the thickness of a medium guitar pick; Would the pick fit between the string and the tenth fret? If there is more distance between the string and the tenth fret than the thickness of a medium guitar pick, the neck will need to be tightened. If there is less distance or no distance between the string and the neck, then the neck will be need to be loosened.
Repeat step three on the high e string.
Generally, but not always, the truss rod is "lefty-loosey, righty-tighty." Make sure to use caution! If the truss rod nut feels stuck or stripped, stop immediately and seek qualified help! You know where to find it.