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

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.  

"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.