Aircooled VW South Africa

how do you test a coil if it is working 100%?

Easy,
If you can have a respecfull conversation with your mother in law who starts the car while you holding the HT lead with one hand and the other hand on the car to earth ---chages are good that the coil is not working
cheers

Very difficult to test a coil working 100% because it might pass resistance tests on the multimeter but it could fail once its under load.... best way is to get an adjustable spark gap tester and make sure it sparks reliably over a big enough gap at a fast enough speed.

Check under pines electronic ign tread there is basic coil testing info