SteveF's link is very good. But, I fear it will scare most who don't have a rudimentary familiarity with things electrical.
Here is a nutshell test for the minimalist. I took a new Honda condenser out of it's package for these tests. I also took a used one out of the junk box for these readings. They tested the same and I would feel confident using either one in a bike.
1) An ohmeter can be used across the condenser (capacitor) to determine if it is shorted or not. It should read open or infinite, as a shorted capacitor is as effective as a straight piece of wire, albeit more bulky.
2) If it isn't shorted, it may still have some life left. Checking its actual value is more difficult. And, the link above explains that well.
However, you can sometimes tell if it is holding a charge by placing its terminals across a battery (observe polarity) and then watching for a tiny spark when its lead is placed on the body of the condenser. This test is mostly useless in daylight. If you see one in a darkened setting, it tells you it held a charge long enough to cause discharge when shorted out. An indication of correct function, but by no means the rated value of the cap. Time between charge and discharge must be very rapid if you hope to see a spark, as it tends to self discharge or leak its charge across its internal plates. It is far from a perfect capacitor, and you wouldn't want to pay for one of those anyway.
3) Here's a check using a voltmeter and a 12 V battery. Affix your meter leads to the capacitor. Place your condenser on the battery terminals, body to negative, other lead to positive. The meter should read full battery potential. However, when you remove the capacitor lead, the meter display slowly diminishes its reading. My digital meter took about 10-15 seconds to get back to zero. My analog meter did this in 1 second. This test outcome will depend on the circuit loading effects of the meter. But, if your digital shows a slow decay reading, the condenser is still behaving like a capacitor and probably serviceable.
4) If you have an analog ohmmeter, the way the meter deflects can give you an indication of proper condenser function. First discharge the cap by bridging the output lead to the body. The analog meter supplies a small voltage to charge the cap. An analog meter will read this charging condition as continuity until the charge voltage is attained in the cap. The meter initially deflects and then returns to infinite if the cap still has some functionality. Use a high resistance scale for this; 1K, for example.
Some digital ohmmeters can be employed in the same way. But, these often have very low injected voltage for the resistance test and the sampling rate of the instrument and display may not allow observing a perceptable charging action. But, my Fluke meter does, and even has a bar graph showing the charge process.
Cheers,
