If a buzzing sound is coming from the fixture or the bulbs begin flickering, it is time to examine the ballast. According to Cecilia Harsch, an author for Home Guides, there are many steps to testing fluorescent light ballasts, some of which include checking for leaks and using a multimeter.
To test the fluorescent light ballast, turn off the circuit breaker for the fluorescent fixture, and remove the fixture's lens cover. Afterward, remove the bulbs from the fixture. Next, take the ballast cover from the fixture, and replace the ballast if it appears leaking, burned or swollen. Use a multimeter, and choose X1K if there are a variety of ohm settings. Put one of the multimeter's probes into the wire that is connecting the white wires together, and then place the remaining probes in contact with blue, yellow and red wires leading away from the ballast; not all ballasts will have a yellow wire.
If the multimeter needle does not move, the ballast is bad. If good, the needle will move all of the way toward the right. If necessary, replace the ballast and the cover. Finally, install fluorescent bulbs, replace the cover for the lens, and turn the power back on.