the only way to know for sure is to get a machine that measures uv output but they are very expensive. It is a good idea to change it soon you can go a little longer and be ok prob but you never know for sure
if it is a linear fluorescent, you are probably best of replacing it every 6 months like others have said. Also like others have said, the only way to tell if a bulb is putting out a good amount of UV is with a meter which will cost you $150 on up
