They are usually controlled by a coolant temp switch and an a/c refrigerant temp switch. Find one of the switches and jump it. Fan should come on. The a/c switch is usually on the receiver/dryer. Not to be confused with the pressure switch on the receiver/dryer.

Yes, on the receiver/dryer, look for a switch with two pigtails about 4 inches long. (The pressure switch should have two spade connectors on the switch itself. You don't want that one.) Disconnect the pigtails. Jump the two wires (the ones not connected to the switch) and the fan should operate.

Make sure that the ignition switch is "ON" when doing the check. Engine need not be running.