How do you test a bad emergency ballast?

To test a bad emergency ballast, first ensure the fixture is disconnected from power. Then, use a multimeter to check the input voltage at the ballast terminals; if there's no voltage, the issue may lie in the wiring or switch. Next, simulate a power outage by disconnecting the main power while observing if the ballast activates the emergency lighting. If the light does not turn on or flickers, the ballast is likely faulty and should be replaced.