Maybe you can try ssh'ing to each of the computers and running some commands (like "who", "ifconfig") to
get the information you need? But those logins probably include both local and remote ones.
But you'll need to have an account on each of the machines if you want to remotely login on them from the server.
Also, if the machines are on the same network, isn't it easier to just go over to the machine and check who's using it?