In order to check glow plugs and circuit you will need two people (for at least part of it).
1. Setup a volt meter on Volts DC (might have two parallel lines to indicate DC). If there is a range, make sure 12 volts is in that range. Example is, range might say 0 to 20 volts. That would be a good setting.
2. Check battery voltage with meter to ensure meter is working. Put black lead on negative battery post and red lead on positive battery post. Should read about 12 volts.
3. Find glow plugs on top of engine. Should be one for each cylinder. They will probably be under a metal strap tying them all together. You would see small threaded rods coming through the metal strap (called a bus bar). Put red lead on the top of one of the "clean" threads. Put the black lead on a good ground. One good ground is the top of the battery if you can get to it.
4. Have an "experienced" person sit at the controls. Have them turn on the glow plugs and read the meter (don't leave on for more than about 15 seconds). Should show about 12 volts. If voltage is there, then the circuit to control the glow plugs is working. If the voltage is quickly dropping off then your plugs are probably working.
5. If you want to check each plug, you'll have to remove the bus bar on top of the plugs and remove each plug. Once removed, you can check with the same meter. Set meter to ohms (omega symbol). Put it on the lowest setting if it has one (like 0 to 100 ohms). If it is automatic, that is fine too. Touch red and black probe to each other to check meter. Should say about 0 ohms. If that works, then check the plug by putting red lead on small threaded top of plug, and the black lead on the larger body thread. Should have a resistance under 15 ohms (might even be under 10 ohms). If the meter shows like 1M ohm, or just blinks, the plug is probably bad.
Any more questions, let me know.
Safety tip: Do all the above with the engine OFF and COLD.