A digital or analog multimeter is used to test capacitors. There are two types of capacitors used in window AC units — run capacitors and start capacitors. Some manufacturers combine both the run and start capacitors in one unit.. Locate the run and start capacitors. Run capacitors are oil-filled capacitors while start capacitors are electrolytic capacitor; run capacitors are round or oval shaped, silver-colored metal cans, while start capacitors are round, tubular-shaped, black-colored units.