You can find out someone's username and whether they are logged in with the finger command.
"name" can be a first or last name, or a username.
finger Wilson Login name: helper In real life: Arthur Wilson
Mr Wilson's username is "helper".
You may also finger users at remote sites if the remote host has been set up to allow this.
finger Salmon@bigfish.co.uk [bigfish.co.uk] Login name: stephens In real life: Stephen Salmon