Do electrostatic furnace filters wear out? How does one know if they need to be replaced?

I'm apologizing in advance for asking what seems to be a stupid question, but I still haven't found a definitive answer.

It is hard to tell looking at the ones I have if they are actually filtering anything. I wash them and afterwards, residue can be wiped off with a paper towel, though it's difficult to see any dust or dirt on the actual filter parts before washing them.

Electrostatic air filters do have a higher static resistance to air flow than throw away filters, but about the same as a poorly maintained 5" pleated, so the trick is to keep them washed.

Also watch out for any brand electrostatic that has a foam core to it. The foam core can become clogged and a visit to a car wash may be needed to actually get it clean.

If you can't easily see light through the filter after it is washed and shaken out, then take it to a car wash, lay it flat and wash it out. If you have smokers in the house even this may not get the filter clean, better off to just use throw away filters (after all if you were worried about clean air, then smoking in the home kinda negates any "clean air" equation.). Tar from smoking is very hard to get from a washable filter without the use of some kind of solvent.