Let's start with the basics. Your 7.3 engine has 8 glow plugs. I think previous posters have told you adequately how to test them. Before testing them, however, you must find them and perhaps remove them. You'll need a 3/8 deep well socket (I think that's the right size).
The picture below shows two glowplugs, on the driver's side of the engine. They're the things with the white connectors. The red caps are on the injectors. On the right you can see the valve cover with the engine decal. Knowing where those two are, and what they look like, I'm sure you can find all the others.
You want to avoid, at all costs, breaking the tip off a glowplug as you extract it. Post back to the forum if you meet any resistance when unscrewing them.
