When it comes to testing capacitors, frankly, I'm in the dark.
Which is why I was and am so grateful for the following information about testing capacitors, from an anonymous visitor, who took it upon him/herself to provide this information for me to share with you.
The information about testing capacitors is copied here exactly as received. He/she and I both hope you find it useful and helps you resolve one of the many reasons why, from time to time, your air compressor will not start.
"I'm probably too late to help the original poster, but someone might find this thread and be thankful for a little insight.
There are various ways to test capacitors, but for your average handy person, there's an easy way to tell whether they are good or bad.
Generally, capacitors themselves fail in one of two ways: they become an open circuit, or a dead short. When they're a dead short, usually they trip a breaker or explode, so most likely it's not dead shorted. I'll tell you how to test it after letting you know about the other "capacitor failure" that's just as common as the capacitor itself.
The start capacitor for one model of a a Bostitch air compressor is shown below.
That's a connection failure. Look closely at the terminals where the capacitor connects. Are they rusty? Are the push-on connectors loose? Are they discolored, like they were overheating? Is there melted plastic around the area? All those things could mean a bad connection... I even had one where the terminals and wire connectors were fine, but the rivets that held the terminal (push on tab) on the capacitor were rusty enough that there was no connection. The electrical test to the rivet showed a good capacitor, but not to the connector.
You Need At Volt Meter / Ohm Meter
OK, to check the capacitor you need a volt meter/ohm meter. Do not touch the terminals of the capacitor with your fingers or metal objects before you're sure there is no charge on the capacitor! It could have up to 1.4 times the voltage that the motor takes (the actual peak voltage reached by a 120V ac line is actually over 170! 120v is just the equivalent DC voltage, or the root mean square of the sine wave... but that's another story).
Step one - CHECK VOLTAGE on the capacitor with your volt meter set on DC Volts. Make sure to check all the terminals on your capacitors, and push your probes hard enough to dig in through the crud or surface oxidation. Does it read close to zero? Great. If you read that accurately, then it won't melt the tip off a screw driver that you will short across the terminals to make extra sure. Again, push it hard enough to make a good solid contact with both terminals, but no other metal (and make sure you are not touching the screw driver or anything that could lead to a shock). If you were wrong about the reading, or didn't have the voltmeter turned on, there will be a really strong "zap!" - a spark that might melt a chunk out of your screw driver.
Please go here for more information on testing capacitors.