Well caps do serve a useful purpose, even in automotive applications. The one big thing they are good at as stated already is power storage. Thats why they are used so widely in audio applications, they fill the gap where the battery/alternator cant keep up, but they do have limits and as mentioned you can get one big enough that it could potentially put too much of a load on the alternator.
The next thing they are good at is filtering AC noise out of an electrical system. In a car that is powered by DC that is a really good thing, particularly with all the AC noise that can be introduced into the system by various components. Anyone ever hook up an oscilloscope to an automotive system? It can be quite loaded with AC noise.
Usually with capacitors, they are marked in some way. Smaller circuit board one have a lead that is longer than the other or marked on the part itself in some way. In larger ones, Ive seen + and - signs stamped into the terminals themselves.
Another way to determine it (someone already mentioned the test light) would be to also try using a multimeter set the measure DC voltage. The red lead is positive, the black is negative or ground. Touch the two terminals, when the voltage reads a positive voltage (i.e., not -12 or something like that) then the terminal the red lead is touching is the positive terminal.
Capacitors can recharge themselves on the static electricity in the air. In electronics school we were taught to keep a resistor across the terminal of large capacitors. At that time a large capacitor was 200,000 micro farads. I can only imagine the charge full farad or multi-farad capacitors can store from just the static electricity in the air.
Ive seen a 100,000 micro farad capacitor arc pretty darn good from just the charge built up from static.
