How do i measure Ethernet signals on an oscillocope
An oscilloscope is the wrong tool to view ethernet data. Unless you expect signal integrity problems. Then you need the scope.

The best tool for the job is a switch with port mirror and a spare network card on a PC to run pcap with Wireshark.

You can also use a ethernet hub, but you can't buy those anymore. So unless you have from the 90's, you're better of buying a managed switch/router with port mirror capability.  

eg: Mikrotik RB260GS or RB750r2.