
Hello,
Are you worried that an outsider may be connecting to the Internet? He or she using your Wireless network? Then no need to worry i have some points and trick.
1. First one is try AirSnare - it's a free utility that will look for unexpected MAC addresses on your Wireless network as well as to DHCP requests.
2. Second option is that you open your router's administration page (using the 192.168.* address) and look for the DHCP Clients Table (it's under Status > Local Network...