you should google Dieselgiant's write-up on testing glow plugs; there is no way I would go through the effort and rip my hands up taking the plugs out unless i had reason to suspect them.
Then i would just go ahead and change them,

Do Not Use Autolite or other garbage from you local parts store, even if it is cheap, unless you want to do it all over again in a few months!!! don't ask me how i know...

Phil has em for 10 bucks a piece for nice German ones, that is the way to go!

__________________
Think Alternative Energy!
300CD '80 (now gone but not forgotten...)