If your test meter can’t read low resistance (1 to 6 ohms), or you don’t have a multimeter? Do a continuity check.
A continuity check will test if there is an open circuit (check if the glow plug is broken internally). You can check continuity with a cheap & simple ‘buzzer’ or ‘light probe’ tester. If you don’t get a ‘buzz’, or a glow from the light, (the glow plug is open circuit) and is defective!
It’s very easy to check, even if you don’t have a test meter (multimeter). To get the best results, remove the glow plug before testing.
More detailed information can be found on this page: Diesel Won’t Start When Cold – How to Fix Glow Plugs: (Opens in a new window / tab)