Are the encoders single ended or differential?
that is, is there a single A,B and perhaps Z wire or are there a Pair for each? (A,/A B, /B and Z, /Z)
The encoder signals can be checked with a voltmeter or perhaps more easlily with a LED and a resistor
For single ended signals the LED anode should connect to 5V through a say 300 Ohm resistor and the cathode to the encoder signal
For differential encoders the LED+resistor should be connected between the A and /A or B and /B signals
Both the A and B signals should toggle when the shaft is turned slowly