Lately we have been asked a lot about how to determine what/who is connected to their wireless internet. Most people become curious when they suspect someone is using their connection without their permission. Our first suggestion is to verify they are running WPA-AES security. If they already are then we suggest changing your password to see if that helps solve the problem. Second you could also check your connected devices within your router's software. The best thing you can do is make sure you are running WPA-AES wireless security.
For more in depth information check out this post over at Lifehacker