Used M.Logans-What to look for?

I have several options through private owners near where I
live with the SL-3; the Ascent and an Aeon used and I just din't know what to look for to be sure that they are not
damaged in any way since I have no clue in these so called
"electrostatic" speakers. How do you tell the condition of
the diaphragms or the panels? By the way your inputs so far
have been very interesting and helpful and I thank you.

I already know I'm a STAT guy. I have auditioned the ML's
for the first time a couple of weeks ago and fell in love
with their most life like sound. But I have read a few ads
where the seller admits that in some areas on the panel of their used ML's, the sound is much lower than other areas on the panel of the same speaker just to be honest in their
full disclosure and also another admiting that their speakes had new panels put on.
This obviously made me wonder if there is something I could
look for or listen for in order I don't come home with something I end up having to repair or replace. By the way, why would anyone need to change the panels of their ML's? Is this something that needs to be done when they get old and start fading in the sound in some areas on the panels?

Just picked up a used but mint 99 model SL3's a couple of days ago. Thanks to you I've learned a few things about these speakers.