If you have nmap installed, that might be a good option. Scanning a /24 segment is reasonably fast (not lightning fast, but tolerable), but I wouldn't want to try any massive address space. OTOH, there might be reasons why that doesn't find anything, depending on how paranoid the original network set-up was.
You might also want to try capturing some data. Wireshark would be my normal choice, but as that's a GUI app, something like tcpdump is probably more appropriate.