Is there a way I can test/see the thermostat and flaps in my 2L '75 Kombi without pulling too much apart? Since I installed my oil temperature dipstick, I've noticed it takes a good 20 minutes to get to 100 degrees, which seems like a long time - so maybe my thermostat is bad and/or the flaps are stuck open.

To test the thermostat, unscrew it and pull it out. Boil the kettle and dunk it into a hot glass of water. If it expands, it's working.

With the thermostat out, pull/push the wire that it connects to. If it move, the flaps are working.

Now turn the engine on. The cooling air comes out at the back of the tin. Place your hand there, you should feel air flow.
Pull the wire again, and hold it. If the airflow stops/slows, then the flaps are definately working

Put the thermostat back in and adjust to spec.
With a cold engine, turn the car on. Place your hand in the airflow again. If it is quite small then the system is working.

Cheers
Tnate