Article: How To check your action like a repairman does
Is your guitar hard to play? Many factors work together to determine how easy or difficult it is to press the strings hard enough to get clean, undistorted notes without buzzes. Some factors are easy to adjust, or compensate for, while others are difficult or impossible. Before proceeding, let's look at the gross geometry of your guitar. Sight along one edge of the neck, from the peghead end toward the body. Now mentally extend that line further. If it dives into the face, I'm not surprised if the guitar is hard to play. If you rest a straightedge on the frets and extend it until it reaches the bridge, it should make contact somewhere near the top of the bridge. Be aware of humps or dips in the fingerboard too; they will limit how low the string action can be set. Is there a gap at the back of the heel, where it meets the body? The neck could be loose- if tuning stability is also a problem, best let someone with experience look at it. If everything is solid, but the action is still high, we need to determine where the problem is, so let's be systematic.
Start at the nut. If the action seems very stiff when you play first position chords, this could be a major factor. Fret each string in turn just past the second fret. See illustration. There should be the tiniest clearance between the string and the first fret. If the string contacts the first fret when checked like this, open string buzzes are likely. If there's more than a hair of clearance, the nut slots are too high. We use fine files to deepen these, making sure the slot bottoms are rounded and smooth, and taper down toward the tuning machine side of the nut.
Next, let's look at the relief over the length of the neck. Fret the 6th string at the first fret, and also over the heel, at about the 13th fret. Using the string (which is under tension) as a straightedge, check the clearance between the bottom of the string and the 7th fret. There should be a small gap, about 1/64th inch. Much more than that indicates the truss rod, if one is present, may need to be tightened. If you do this, turn the adjusting nut about 30 degrees, then check it again. If another 30 degree turn still doesn't change that gap, stop. Better have someone who has experience look at it.
OK, let's say the nut and truss rod are fine, but the action is still high. Your only recourse at this point is to lower the saddle. Remember that cranking down the truss rod more won't help; you'll wind up with the unfortunate combination of high action and buzzes, too. Without fretting any strings, measure the gap between the strings and the 12th fret. Measure from the top of the fretwire to the bottom of the string. On the high E string you should get 4 /64ths to 5/64ths. The 6th string should have a 6 to 7/64th gap. However much you need to lower your current string height at this point, remember you'll have to remove twice that amount from the saddle. How much you can safely take off is difficult to predict; if your fingerboard is wavy or if there are loose frets, you won't be able to get nearly that close. (If you have a 6 inch steel ruler, lay it edgewise on the frets to see if it rocks) If this is an inexpensive guitar with a plastic saddle, you don't have much to lose. To be safe, see if you can get a replacement saddle before altering the original. On the other hand, if this is an old Martin with a glued-in ivory saddle, don't screw with it! Let someone who knows what they're doing look at it. If the saddle pulls out, measure and mark a line on its side and sand off material to the line. Reassemble, and try it out! By the way, it's better to have to do this twice than take off too much and have to scrap the saddle.
To recapitulate, action is adjusted in this order: nut; truss rod; saddle. Even if you don't have the nerve to do the work yourself, you'll be able to explain the problem more clearly to your repairperson, and you'll understand what they're going to do.