I've never made my own galvo scanner, but I've used them. General Scanning galvos use capacitive sensing, and Cambridge Technology use optical feedback. Both methods work well, especially if you use differential sensors. The way the General Scanning sensor works is that a 2.2MHz sine wave is applied to the plate on the rotor, and the current splits between the two stators according to the angle, as it determines the area of overlap. Rectifying and subtracting the currents gives you a position signal.

Be warned that designing the control loop is very difficult, and if you get it wrong you will melt the galvo coil.