I've been in a similar spot in the past, luckily everything was set up properly so i did not have to change anything .. just find out what was connected to what and what machine was used for what purpose
I used some network scanning tools that draw a graphical map of the network. "the Dude" or spiceworks are good ones that are both free.
there are a bunch of other tools out there that can give you detailed info on what is connected to your network.
following the wiring is not easy if your racks are not very neatly organized ... it wil be tough untill you sort everything out and how it is all connected .. best of luck