Depends heavily on the terminal emulator. If it's one in the truest sense of the name, they'll be connecting as though on a directly connected terminal - so no IP. In that case, you'd need to look on the emulator to find the IP.

If you mean a terminal emulator like a piece of software on their client machine and they come in over IP and talk to something that doesn't use login. Find the relevant process and see what IP's it's got open connections to.

Or have I misunderstood the question?