Here is a free download program and shows parameters etc for 0M http://www.cadem.com/ncnet/ncnet_dncdetails-41.htm
IIRC that should just be the overtravel switch, make sure you have the X- , you can also look at the diagnostic input status bits, the overtravel on Fanuc are dedicated inputs, i.e. they are pre-defined, but I do not have my maintenance manual here.
If you cannot get the original OEM service manual, you should get hold of the Fanuc Connecting, Connection and maintenance manual, keep a watch out on ebay.
No third party documentation on Fanuc 0 AFAIK.