mpj68
In order to check the heater you have to remove the dishwasher from the cabinet to get the access to the heater terminals.
Remove the power to the dishwasher first.
Do not unscrew the plastic nuts holding the heater. If you'll do it then you must to replace the heater, otherwise it could be a water leak.
Remove the wires from the heater terminals and check the resistance across the terminals. It suppose to be between 10 Ohms to 35 Ohms. If your meter reads a different resistance the heater should be replaced. The part number for the new heater is
AP3884745
If the heater is fine then you have to check the hi limit thermostat which is wired in series with the heater. It looks like a small tablet with two terminals and is attached to the bottom of the dishwasher on the right side (from the face). If it is open then it has to be replaced. The part number for the new hi limit thermostat is
AP3010530
For your references here are the break down diagrams for the Whirlpool dishwasher
Model GU1200XTLQ1
Good luck.
Gene.