The best way to tell is to have a UV meter and monitor the output. Otherwise, it's recommended to replace them after about a year. Their warranty is 6mo. Are you thinking about the MVBs or the Metal Halide?
Every Mega Ray I have had doesn't even last a year. It conks out before then........flickers off and on for a few days, then just quits working. In fact I just put a spare Powersun I had in my BD's enclosure until I can get another MegaRay because hers just quit. Really disappointed in the life span of them for the cost. But they seem to work because my 11 year old girl is as healthy and active now, as she was when she was a lot younger.
Mirage came out of brumation on April 26. He was doing great. On May 2 he started acting funny. We just redid his tank, and he keeps going into one of his hides. He just lays there. He shows no intrest in food. HELP!