If you want to check them electronically, identify the A & B channel connection and preferably check with an oscilloscope for the Two square wave quadrature signals, they also may have a complement signal for each channel.
If you inspect the glass scale or want to clean it, use lint free cloth with denatured alcohol.
One thing to be aware of if they are Heidenhain scales is that they may be low level sine wave signal that requires a special interface to square up the pulses to TTL level.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.