If you poke around your device's administrative interface, you'll see a list of DHCP clients. That'll show you who is connected to your wireless network.

You should be able to log into your router and view the DHCP Client list. I don't know what brand router you have, so I don't know exactly what the process will be to get this information. Check your manual. (On preview, what cmonkey said.)

While you're at it, if you haven't protected your network at all, you should at the very least restrict it by MAC address, change your SSID, and turn off SSID Broadcasting. Usually leechers just jump on whatever open network they can find, so this is usually sufficient to "close" your network.

My broadband is running like a dog tonight. Perhaps some DNS stuff? Google is the worst.

Use toast.net to see if you're getting bandwidth or not.