There is no writing on the board?
You have a Z+, Y+, and X+, inputs that can be used for whatever you want.
Easiest is to figure out what board it is, and look for the manufacturer schematic to determine what IO on the atmel they connect to. Then assign one to Z probe.
If you can't find manufacturer schematic you'll have to trace it out yourself or read all the IO, change the switch state, read again, and look for which bit changed.