With the command w, find out who is logged onto the system and what each user is doing. For example:
tux@mercury:~> w 14:58:43 up 1 day, 1:21, 2 users, load average: 0.00, 0.00, 0.00 USER TTY LOGIN@ IDLE JCPU PCPU WHAT tux :0 12:25 ?xdm? 1:23 0.12s /bin/sh /usr/bin/startkde root pts/4 14:13 0.00s 0.06s 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.