It is worth a little investigation before going to the trouble of replacing the ballast. If you have a digital amp meter in your system and can determine the actual power consumption (by comparing system amps with the light on vs. off), that would tell if the ballast is operating at the correct level.
Alternatively, if you have a decent digital multimeter, you can disconnect the light and measure the amps directly with the meter. If the amp draw on high is around .75 amp to .9 amp, then the ballast is likely okay. It's best to allow a couple of minutes of warm-up time before reading the amps. If the amp draw without a bulb or a bad bulb is at .10 amp the ballast is okay. If you get no reading or a reading much over 1 amp, that would indicate a bad ballast.