I bought one of these cheap Galvo Kits from the internet.

And also a Hantek 6022BE USB Oscilloscope for getting started.

The set includes a power supply with +15V GND and -15V output, two driver boards and two of these galvos.

After removing the backside cover you can see an IR LED, two photodiodes and a small piece of sheet metal connected to the shaft of the galvo. Depending on the galvo's orientation and therefore the sheet metal's orientation the IR light hits more of one and less of the other photodiode which gives a quite precise position feedback.

I checked continuity and here is what I've got. Each of the photodiodes is connected to its own pin and to GND.

The next thing will be finding a way to create a control signal for the drivers.