How do i test capacitors?

Charge thru a resistor to the working voltage. Choose a resistor so RC (where R is the resistance, C is the capacitance, and RC is the time constant) is workably large. The final voltage should equal the applied voltage - IR, where I is the leakage current. The rate of charge will give you C ( if I is large you will need to correct for that ) This ignores the burden of the meter which is probably above 1 meg and for a supply cap probably does not matter.

Use a DMM with capacitance measuring. If you don't have one, try charging and discharging the cap through a resistor and measuring the voltage curve.

Even though this is a very old question, I feel like I have something useful to add here, so I will answer it anyway.

The standard, common way to test a capacitor is to use an instrument called an LCR meter. This is a device similar in appearance and usage to your standard multimeter, but which measures inductance, capacitance, and resistance (hence "LCR"). LCR meters measure more figures of merit as well, however; you can use one to measure the equivalent series resistance of your capacitors, which is very important for power supply filter caps, and a better indicator of a failing electrolytic capacitor than capacitance alone.

Some, though I don't think all, LCR meters will even tell you the equivalent series inductance of your capacitor, self-resonant frequencies, and all sorts of other useful and useless data.

Unfortunately, good LCR meters don't come cheap. Low-end LCR meters (from reputable brands) are generally more expensive than equally low-end multimeters. A quick google search shows the Agilent/Keysight U1733C priced around $600, and even the cheap Extech brand one is over $200.