If you suspect someone else is using your connection, you should check the DHCP table in your router's interface (refer to the manual to know where this function can be accessed). If you're the only one, it should look somewhat like this:
If someone's "stealing" your Wi-Fi, you'll see one or more additional entries:
Quote:
Computer name - IP address - MAC address - Lease time
mike160 - 192.168.1.100 - 00-09-8D-E2-57-29 - Forever
stealingurnet - 192.168.1.101 - 00-06-A4-54-CD-6A - Forever
The best solution remains using WPA encryption and a strong password, I'd say.