Using an analogue ohmmeter (leads just connected to the points lead and the distributor body) and finger-twiddling to get a useable reading, which happens at a surprisingly low speed.
Circuit to use a dwell-meter. If you have changed the gap i.e. changed points you will need to check the timing once it is back in the car, even if you made marks on distributor and block, but using those marks will be close enough to start the engine and use dynamic timing. If you are fitting a spare dizzy because the original has failed on the road you will need to static time it first.
Gunson's TestTune dwell scales - percentage at the top, go/no-go sections for electronic ignition, points ignition, and petrol injectors, and a 4-cylinder degree angle scale at the bottom. Note the electronic ignition section is used for variable dwell systems where the dwell angle/percentage closed time varies hugely with engine revs from 8% to 60%, whereas points (all types) vary from 40% to 60%. Lots of other useful ranges:
A test-rig to check centrifugal advance: Basic box to hold the drill
Clamps to hold the drill steady (coil and plugs were so I could drive a strobe light to measure curves)
Distributor drive dog (the blue thing is a right-angle gearbox to drive a degree wheel for measuring curves)
Detail of drive dog, simply a brass plumbing fitting with a slot cut, a bolt through with a nut on the back, ordinarily you would clamp the thread of the bolt in the drill chuck and drive the distributor directly.
Distributor clamp-plate screwed to the end of the box, shaft engaged with the drive dog. The tongue on the distributor is slightly offset to one side, so cut the slot wide enough so the two can be engaged either way round without wobbling as it is rotated.
Rig in use to measure dwell on a dwell-meter. The coil and plugs with a degree-wheel fitted to the top of the right-angle drive enable me to plot centrifugal advance curves as well.