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

I own Quad 2905's and I had to make repairs. Other than listening to them to see if they sizzle, pop or sound distorted in general, you'll have to take them apart. Two main things to consider are burns in the material or board separations from broken glue joints.

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?

"Is the old, top of the line still good vs newer versions?"

Assuming a well cared for and proper functioning used set, the older Summit will unquestionably sound significantly better than a comparably priced new model, if and only if paired with an amplifier that can drive it to its potential and placed properly. 

I believe the panel reliability issue was due to the film coating process on older models.  Summit uses the same panel as Summit X and has the newer electrodeposition process.  There have been other improvements in later models, but I think panel reliability should not pose a problem. $3k seems like a good price if they are in good condition.  As a reference I sold my newer model Ethos for $3k also.  If I had the choice I would take the Summits over Ethos any day and definitely the Summit over brand new bottom of the line ESL ($2.5k) or ESL X ($4k)

I have a pair of ML Summits I have used for years. The panels were never an issue. I did have trouble with the woofers however and eventually ended up replacing all four and never had an issue since. ML is also great at supporting their customers. If you can get a pair of the Summits for $3K I would put that buy in the steal category.