Radiator fan

I was just wondering when is the raditor fan supposed to come on... my car almost reaches 100 when I'm in traffic after 10 minutes or so and I'm guessing it could be because of that....is there any way to test it?

Also I've tried to pull over and see if the fan worked and it was just sitting there idle...temperature was at around 70-80ish (dunno if temp has anything to do with it but figured I'd mention)

Thanks!