How to Check Your Guitar’s Action at the Nut
One of the most frequent types of work performed on a guitar is an adjustment of the action (height of the strings above the fingerboard) at the nut and saddle. In most cases, this will follow a truss rod adjustment, which will ensure that the issue is not with a bowed neck. To check for ideal action at the nut, depress each string in turn between the second and third frets. Look for just a hint of daylight between the bottom of each string and the top of the first fret to indicate that the depth of each string slot is accurate. If there’s no space, the string will buzz against the first fret when played open. If there’s too much, the action will be high, the intonation will be off, and fretting will be more difficult.
Excerpted from Acoustic Guitar Care and Maintenance: Complete Edition