Hi Gary,
What is the make and model of the router?
- There are multiple ways where you can check if some one is accessing your Internet.
You can go to your router home page and see connected devices. That would tell the name of the Mac address/IP address of the connected devices. If all the MAC addresses are known to you then you are safe, otherwise wifi has been hacked. Best option is to enable MAC binding on your router wireless page. This way any new device would not be able to log in on to your wifi device even if your password is known to him.
2. By opening command prompt and typing ipconfig /all Will display all the devices connected on your network. I would suggest you to setup a password to your router to make sure that others will not access your Internet. Also refer to the links: http://windows.microsoft.com/en-in/windows/set-up-wireless-router#set-up-wireless-router=windows-8http://www.microsoft.com/surface/en-us/support/networking-and-connectivity/how-to-find-your-wireless-network-password Please post back the results and we will be happy to assist you further.