I assume that you don't have an ammeter, particularly a clamp on. Therefore, you will have to be nontechnical with it. On my 6.0 it is on the pass side, rear. Very easy to get to from underneath (right above the starter), but I have a 350 so I sit higher. Let it run and after a while put your hand on it, it will be warm. While you are under there, make sure that the cord hasn't been damaged by rubbing (it has plastic tubing over it) or come off the heater (my old trucks heater cord was removable). It will not always "pop" when plugging/unplugging it. Check for a connector behind the bumper where the cord from the heater is changed to regular rubber cord. Maybe it came loose. If you have access to a volt/ohm meter, the correct reading on ohms is around 14.4-15 ohms between the 'two prongs', for a 1000W heater. Not the round one, that is the ground. Each prong should read nothing or infinite to the ground, which is displayed on a digital meter with a "1".

There are all kinds of ways to be nonscientific about this. Perhaps others will post some. Good luck.

Matt