[header] How to test fluorescent bulbs [title] Look for dark areas at the base of the bulb. [step] When bulbs age and have been used for a long time, brown spots will appear. If darkness begins to form at the ends of the tube, the bulb may be near its end. 
While they may still light, they are wearing out and will die soon. [substeps] If the darkness only forms on one end of a tube light, flip the tube so the darkened end is on the opposite side of the fixture.