I have a Toshiba Satellite A45 S-151, and the screen on my computer is out. I can still see things but faintly but im not too positive if it is the fl inverter or the backlight. Or what. What is the problem?

Take it to a dark room and turn the lights off - if you can still see something in the dark, it's the backlight. If you can't, it's the inverter.

Well Im not sure how that is accurate, but I took it into a dark room and I saw nothing. So it would possibly be the fl inverter?

In my experience, that's a very accurate test. So yes, that would very likely be the FL inverter. The FL backlight would die like any other FL lightbulbs - they'll appear to dim down over several months rather than just complete die one day. And realistically, an FL inverter generally costs about $20 or less - well worth trying before anything else.