With the command w, find out who is logged onto the system and what each user is doing. For example:
tux@mercury:~> w 16:33:03 up 3:33, 2 users, load average: 0.14, 0.06, 0.02 USER TTY LOGIN@ IDLE JCPU PCPU WHAT tux :0 16:33 ?xdm? 9.42s 0.15s /bin/sh /opt/kde3/bin/startk tux pts/0 15:59 0.00s 0.19s 0.00s w
If any users of other systems have logged in remotely, the parameter -f shows the computers from which they have established the connection.