Glow plug light and the Sears $29 Voltmeter

I could see this one coming. 85 300D, 230K. Glow plug light took longer and longer to go off prior to starting. Cold here in N. Cal but car starts up just fine. Now glow plug light won't go off at all untill after engine starts.

Researching the threads suggests that it's time to do diagnostics on the glow plugs and relay. Will the Sears $29 voltmeter do the job?

Further research indicates that just leaving it alone as long as the engine starts is OK too but my Virgo/anal personality wants to get it fixed.

Thoughts, suggestions on the Sears meter appreciated. Happy new year to all!