A residential air-conditioning system, minus furnace or air handler has 3 main parts. Those are the evaporator, the condenser, and the line-set. The evaporator, or "A" coil lives over the furnace or air handler and picks up the heat from your domicile. The line-set carries the refrigerant (which also carries the heat) to the condenser. The condenser also houses the outdoor coil, fan assembly, a switch, capacitor or two, as well as the pump to move the refrigerant. This pump is called the compressor, and it not only moves the refrigerant around, it also moves oil around in an effort to keep itself cool and wear free as possible. If the oil becomes contaminated from moisture, the result is that it may form an acid. This sounds ominous, and it is.
There are three windings on a compressor: start, run and common. All of them need to be insulated from ground to allow it to run through the bath of oil it's in, or it will trip the breaker or blow the fuses. Acid attacks the insulation on these wires and allows them to ground out. The condition of this insulation is measured in Meg-ohms and measured with a meg-ohmmeter. The lower the number, the poorer the insulation and more likely the compressor will fail.
Again, you can spend a lot of money on one, but these are difficult to read and even harder for the customer to understand. I am shamelessly endorsing the Supco M500 for its compactness and ease of use. At about $125, it's fairly inexpensive. The leads are stored inside the meter and it has a soft case. It fits in a tool bag with no problem, so it's ready when you need it. You can hook this up to the compressor lugs, one at a time with the black on a convenient ground. To measure, push the button and read the scale. If one of the green L.E.D.s lights up, the compressor passes the test. This doesn't mean the compressor is "good" but that you need to investigate further. If the one of the yellow L.E.D.s light up, the compressor is failing. If the red light is on, the compressor is junk.
An alternative to tearing the condenser apart to reach the lugs is to use the wires and ground to the frame. You also have the added benefit of not breaking off one of the lugs trying to get the leads off. If you're unfortunate enough to do this, it's "bye bye compressor".
The meg-ohmmeter is a useful and necessary tool in evaluating and diagnosing compressor failure. With the price of equipment today, customers demand accuracy as few are willing to pay for maybes. Even on 20+ year old equipment that needs to be replaced, most want to wait it out a year or two. A meg-ohmmeter is essential in reinforcing your argument to replace, or repair the equipment. It also helps provide you, your employer and the customer peace of mind. With all of this to consider, you can't afford to not have this in your tool bag. Maranatha!