We will be Using one of the most popular and Effective Tool Called Who's On my Wifi. The Best part is its available for Free. And it Does its Job Quite well.
After Downloading the Tool, Simply Install it like you install any other Software. Now Launch the application and click the Continue Setup button and you’ll be guided through the process.
Follow the instructions, starting by clicking the Scan Now button. You’ll have to wait for a little while as your network is scanned and connected devices are detected.
You will then be Shown the list of Devices that are Connected to your Wifi, Then you may Analyze and identify which device you trust. After its Done, just Click on save changes.
You Can keep the Program Running the background so that everytime someone tries to Connect to your Wi-Fi, You will Get the Notification instantly. It will Keep you Informed every time.
Method 2: Network Admin Method
Your router's administrative console can help you find out more about your wireless network activity and change your security settings. To log into the console, go to your router's IP address.
You can find this address on Windows by going to a command prompt (press Win+R then type cmd) and then typing ipconfig in the window and press Enter. Then find the "Default Gateway" IP address.
Next, type in that IP address in a browser window. You'll be prompted to login to your router. If you haven't changed the default settings, your router documentation will have the login information, which typically uses a combination of "admin" and "password" or blank fields.
All routers are different, but once you're in yours, you'll want to look for a section related to connected devices. This could be called "Attached Devices" (Netgear) or "My Network" (Verizon routers),
It should provide a list of IP addresses, MAC addresses, and device names (if detectable) that you can check against. Compare the connected devices to your gear to find any unwanted users.
So these were the 2 Methods i wanted to share to make sure no one Steals your Wi-Fi and yu get all the Speed from your own Wifi. If you have any other way to check it please feel free to Comment Below.