Martin Logan electrostatic speakers, How do you know if the panels are good?

Use your ears, use good common sense, go forward confidently.
What else can you do?

In the older ML panels, keep the volume moderately low, hold your ear close to the panel and listen to the entire panel top to bottom. In a healthy panel the sound volume should be consistent throughout the panel, especially from one grid to the next lower grid on the panel.
In the MLs, you could hear popping sound only when the panel is extremely dirty or dusty and needs a good cleaning. If the speakers are kept out of direct sun, high humidity, and cleaned/vacuumed regularly, the panels could last 20+ years without a problem.  

So here is another question.  Is it worth considering older, used to be high end Summits or should I consider getting newer speakers?  The ask is 3k for the Summits which isn't far from simply getting newer , lower end models.  Has there been an improvement in quality over time?  Is the old, top of the line still good vs newer versions? Thanks for the answers so far.  I'm trying to both educate myself and do some due diligence.

If the subject was "cars," how would you answer
these questions?

You might ask this question on the Martin Logan Owners Forum.