Are you trying to heat or cool? If you think it may be a capacitor, shut off power to the unit and visually inspect the capacitor(s). If they have an oily substance on them or look to be blown up, they are obviously faulty. A recommended way to test capacitors is to find microfarad values with a multimeter. You may also want to test the amp draw from the compressor. It may be nearing to draw LRA (locked rotar amps). Generally if capacitors are starting to fail, the compressor isnt far behind.