How can you tell if a flourescent light is blue/purple?

Blue tubes will be labelled "actinic"(I use these for my reef tank), if not, ask someone at the store to find you some actinic bulbs. purple tubes are ultraviolent lights wich, go figure, put out a lot of UV light wich will be giving your fish a bad sunburn every time the light is on, and of course, will shorten thier lifespan, so should not be used over fish.