Visual inspection says a lot.. If the condenser's can bulges out of there is any leakage, toss it. Meters that accurately diagnose capacitors are expensive. So the short easy answer is to substitute a known good condenser.
I've got a DMM (digital multimeter) that has a capacitor checker.. What it does is fully charge the condenser and then measure how much capacity it actually has.. Although this says the cap is not dead-shorted or has some major malfunction, this doesn't determine if the cap is leaking more than it should. (Charge in a condenser always leaks cross the plates but only at some acceptable rate in a good one..) Another thing is the meter can only charge the cap to a low voltage.. not the voltage is sees normally. Breakdown voltage is important. An oscilloscope to test a cap while it's in the circuit might be needed.
If you are suspect misfires, how about hooking up a inductive timing light .. the flashes of light would not be steady BUT the plug would not show a wet, fouled condition.