I assume you all are using electrolytic capacitors, the way to check them involves have a digital multimeter that has a capacitance scale or with a capacitance meter. The steps would go like this:

1) remove cap from car.
2) with a jumper wire or screwdriver short both leads together for a few seconds.
3) attach meter leads (with meter on capacitance scale) to the leads. Pos. to Pos. Neg. to Neg.
4) depending on the meter being used, you will get a capacitance reading in pico or micro farads.
5) compare this reading with the caps rating. Anything that is more or less than 5% from the rating, in my opinion, is a bad cap. Some will tell you 10% is ok, and might be, but I like 5%.