Command Prompt can let you know if someone is connected to your Local Area Connection and using it. Just follow the steps:-
1) Open your browser and visit http://192.168.1.1 or http://192.168.0.1 depending on your router.
2) Find the tab that mentions “Attached Devices” or something similar.
3) Find the computer name, IP address and MAC Address (sometimes called Physical Address or Hardware Address) of your computer using the previous trick.
4 )Compare it with those displayed by your router in Step 2. If you notice some strange devices, then your neighbour has been sneaking in on your internet connection and it is best to add a password.