Morris Minor Owners Club

I have a collection of old regulator boxes.
Is there a simple off the car method of checking which ones work and which are u/s?

Not really - other than inspecting the moving parts to make sure they CAN move, and the contact points to make sure they CAN contact!

I was wondering if I connected 12V across the F and D terminals, replicating the dynamo connections, would the contacts open and close? I obviously don't want to swap each regulator with the one on the car, especially as some boxes are spade and some are screw terminals

That won't work - you will need to run them with the dynamo - but no need to interfere with the car electrics - just make a simple test rig/board.