Easiest way to check a glow plug is use a test light.Clip the lead on the + terminal,yes positive,and then touch the test light to each glow plug.If it is good the test light will glow.Make sure you are touching the small centre prong on the glow plug,and not the body of it,or it will cause the light to come on,when the glow plug may actually be bad.This is the same as using a continuity tester,just much easier.The light will come on when grounded,so when you touch a good glow plug,it has a ground,and the light comes on.
BTW -as someone mentioned previously,anti-sieze compound is a must,preferably the silver colored,or it will never come back out.