"How do I test voltage?"
Well, it depends whether you want to check voltage or resistance; Dr. hex suggests checking the resistance of the Hold winding, by selecting resistance on the meter, and (with the power Off) placing one test lead on the 'front' end of the coil (Blue-yellow wire for the right side) and the other on the end of the Hold winding (Orange-trace wire). A very low reading could be caused by the diode conducting rather than the winding itself, so in that case change over the two leads to eliminate the diode. As above, the winding should read at around 150 ohms without the diode.
This is a parallel-wound coil, so checking at the middle and tail-end terminals won't show a direct measurement of the hold winding, but the one of the diodes and the hold winding. Incidentally, Williams practice at the time was to fit the flipper coil with the terminals away from the stop end, to reduce the incidence of broken connections.
As to measuring voltage, then I recall Andy mentioning previously about this that each terminal should show around 70v dc without the flipper operating. Set the meter to dc volts, and put the -ve/com/Black lead on ground, such as the braid inside the cabinet. Then touch the +ve/Red to each coil terminal, with the power On. I suppose that as with a continuity check, the return wire to the Fliptronic board should also show the voltage.