You can check them with an Ohm meter.
If digital, put the selector on 20M
If it is still in the car make sure the lead is disconnected.
To charge it put the black lead to the case and the red lead on the condenser lead. You should see some movement of the meter. When it stops it will be charged. Now switch the leads and put the red lead on the case and the black lead on the condenser lead. You should see the Ohmeter go up in a couple of steps and then go back down as the condenser discharges.
At one time I remember testing small engine ones by charging them on a battery and then grounding them. If they sparked they were good. Not sure if that’s good for the condenser though. You could likely see a jump with a voltmeter if it was charged as well.