Your modem will automatically notify you when anyone tries to connect your wifi, But that is possible only when you have used an easy password or no password, avoid using an easy password or default password in your modem otherwise anyone can connect and steal your wifi data.
Maximum times your neighbors are stealing your internet, so always update your password if you share your password with multiple people then change your password and avoid using any Wifi Password Show app in your device, because they save all your password and display that to the public, so if you are using any such app then remove that immediately and assign a new password.
You can easily detect Wireless Bandwidth Theft using simple tricks and methods, All you need is to follow steps properly.
How to Detect Wireless Bandwidth Theft
Log in to you network router by typing the IP address into the address bar of your web browser. The IP address often is written on the router, but more often than not the address is http://192.168.0.1 or http://192.168.1.1.
Open the list of attached devices. This could be called “Status,” “Device List” or “Wireless Nodes,” depending on your router. You are looking is a list of MAC addresses or “clients” that are using your network.
Verify each device listed as your own device. If there are more addresses than your devices, your bandwidth is being used by another device.
How to Stop Wifi Bandwidth Theft
To Stop Wifi Bandwidth Theft use Scary Wireless Names like c:virus.exe or something like that which discourage user to use your Open Wifi Network.
Find Who is using your Mobile WIFI
If you are a mobile user then try using android app “Who Use My WiFi?” this app will display all the devices connected to your wifi with their IP and MAC address.
If any unauthorized device is connected to your mobile phone then it will provide you with whole details.