ORIGINAL: Eganwp
ORIGINAL: LNEWQBAN
Just don't suspect, test instead.
Besides ChuckW's advice, mixture on the rich end keeps plugs healthy, while mixture on the lean end kills them fast.
Listen for a noticeable drop in the iddle rpm's of the engine when you retire the ignitor; that's a sure sign of plug going bad.
Significant rpm drop when plug ignitor is removed (when engine isn't tuned excessively rich obviously) is also an indication that your plug temperature range is too cold. Move to a hotter plug if it does this on a new plug.
So I take it then that there is no good way to test the condition of a glow plug on the bench. IOW you need to run it in the engine. (That;s the engineer in me coming out. We like to have validated test procedures).