How do you know if someone is tapping into your wireless network that is unsecured?

Usually a router will give DHCP lease for a period of a day or two. If you check your router settings, it'll tell you what clients have connected over the past while.

just use mac address filtering

Those methods are great for detecting/foiling most bad guys but you don't really know for sure unless are running wireshark in the subnet.

As a general rule, you should go ahead and assume that someone is using your wireless if you knowingly leave it unsecured.

(Wireshark is the new name for Ethereal)

If by tapping you mean listening, they could be running kismet and sniffing all your packets, mac filtering or ethereal wont help because kismet doesn't transmit anything. If you can't encrypt it, consider your traffic public.

by tapping, i mean someone using my internet connection.

If by tapping you mean listening, they could be running kismet and sniffing all your packets, mac filtering or ethereal wont help because kismet doesn't transmit anything. If you can't encrypt it, consider your traffic public.

Good call. I hadn't considered that he was talking about eavesdropping.