I find that a cap helps, I put one in my truck, and it really helped to stop the lights from dimming.
A voltmeter might work (I've never tried it), if you put it in series. If you don't have that gimiky LED thing that they give you when you buy it, use a test light.
Ground the negative terminal of the cap, and then attach the test light to a power source (the battery works) and touch the test light to the cap. The light should turn on, when it turns off, your cap is charged.
You only need to introduce some resistance to the circuit to avoid charging the cap too fast, a plain resistor will work, but the test light is nice because it's easy to hold onto and it gives an indication of when the cap is charged.
If you ever need to discharge the cap, use the same test light and just put one end on each terminal, and again when the light turns off it's discharged.