How can I tell if someone is using my internet?

[Mulch]

My internet is running slow and I want to eliminate the possibility that someone is else is using my internet connection.

[Ad Bot]

[Alyx Vance]

Walk into other room and ask if they're on the internet.
If you think its another house than I don't think its possible

[james4115]

there's probably a way to see current connections from your router's settings

[dkamm65]

Do you mean stealing your wifi? Just change the password.

[Altiris]

I think there's a thing an option in your router where you can check, if not just set to how many people can use your connection to 1 and then see how the speed is.

[carolcat]

I think there's a thing an option in your router where you can check, if not just set to how many people can use your connection to 1 and then see how the speed is.

this, pretty much

[Doomonkey]

Get angry IP scanner (http://www.angryip.org/w/Home) and run a class C scan starting at your own IP address. Make sure to search for only alive hosts.

EDIT: You can also probably do this through a tool on your router.