A Question on Surge Suppressors

Hi.

I’m new here so forgive me if this is in the wrong place.

I’m in the process of building some homemade surge suppressors, and I was wondering if someone here could clue me in on what kind of circuitry the pros use to monitor the health/age of MOVs?

Or, is it not really possible to monitor something representing millions of ohms in an active circuit, and those LEDs are really just for show?

Thanks