Quite a few of the older Macs did not have the fan sensors in them. My 12" iBook didn't, but the 12" PowerBook did. If the fan is not running, don't use it unless you are testing a repair or you stand a chance of damaging it. If you know the fan is not spinning, I'd go ahead and replace the fan. They are normally inexpensive, and not an overly difficult repair. Before doing the repair, you might check and see if you happened to get something lodged in the fan. I've seen that happen (rare, but it has happened) and would save the expense. Also check to make sure there is nothing else obviously wrong.

Any circuits that are blackened, capacitors that are bulging or popped, and other things like that are what to look for.

Good luck!