quick test(but be forewarned, this will turn on the check engine light-you'll obviously have to clear it, or wait for it to go away.):
with the the engine warmed up all the way and running, unplug the coolant temp sensor(the one in the t-stat housing). the fan should come on within 5 seconds or so. if it does, it's "probably" the coolant temp sensor. if it does not, you have an issue somewhere with the pcm refusing, or not being able, to activate the fan.
this is something i learned from a tech at the jeep dealership i worked at. he used it as a quick test of the fan motor, relay, wiring, etc.
also, for reference, my '96 would turn the fan on when the needle just passed the 210 mark.