measuring stiffening caps

I believe there are some meters that can measure this, but they're probably only useful for smaller amounts of capacitance, and not big ones used in car audio.  If you need to figure this out, here's what you can do:

When charging a fully depleted cap, (or discharging a fully charged one) it is generally excepted that it takes 5 time constants.  Knowing that, you can use this formula: http://www.design-technology.info/electronics/page3.htm and solve for x.

Take a resistance measurement of a light bulb which you'll use to charge (or discharge) your cap.  The higher the resistance, the more accurately you can estimate the capacitance of the cap, but also the longer it'll take. 

When charging a cap, you place a light bulb in series, and when the light goes out, it's charged.  Now, using a stop watch, time how long it takes to charge the cap to a given voltage (12v).  *Caution (as well as some common sense) must be exercised here, because if you use too much voltage you will damage the cap.  Some caps may have markings indicating maximum safe voltage*  Divide the amount of time it takes by 5 to get one time constant, then use the formula in the web page above to solve for x.

This will give you a good idea of how much capacitance it has.