The way the solenoids work is as follows:

+ voltage is applied to one side of all coils - this is why you need to take care when working under the playfield with power on. As you have measured, 50~80 DC volts is typical.

The transistors take the OTHER side of the coil to ground thus applying the voltage across the coil.

You will measure full DC coil voltage at the connector of the PCB when the game is switched on as the TIP36 grounds this wire to activate the coil.

The FIRST voltage you need to know is the base of the TIP36 - this is leg number 1 (the one on the left looking at the face of the transistor with the writing on it).

It should be ground or VERY low - maybe 0.1V with the ground lead of your meter connected to the braid inside the backbox.