...correct me if I'm wrong, but i think the 88s' duke fan is ecm controlled, therefore, it's possible that you may only have a coolant temp sensor, unless you have the 6cyl, in that case, you have a separate fan switch (next to the thermostat housing, on the lower intake), a coolant temp sensor for ecm (directly below the thermostat housing, orientated horizontally), and a temp guage sender ( corner of rear bank head, directly under valve cover, close to the coil).
You can test the guage one by grounding each lead out, one for the warning light and one for the guage. I'm not sure how to test the fan switch one. You can test the circuit by grounding the lead and the fan should kick on. I can't remember what temp it kicks on, i think it's 220 or something. It takes a while at idle for it to kick in.