If you had given your Wi-Fi password to somebody and later you forgot to change the password, It might be difficult to ask if they are still using it. So ideal way is to check somebody else is connected to the Wi-Fi router.
Here’s the way to find out.
1. Check your Wi-Fi routers page, Normally the ip address to connect to it will be given below the router or along the side. In my case it was 192.168.0.1. Simple way to find that would be to goto your command prompt by typing cmd in run dialog. and entering ipconfig /all
Here you can check for the Default Gateway ipaddress.
Now goto your browser and enter http://ipaddress which you got.
It will prompt for user name and password. This user name and password will be written normally on the router or the manual. Majority cases it will be admin/password or admin/admin etc.
Once you do that you can see the wireless statistics to check the devices connected to your Wi-Fi. Here you can see the mac address matches the mac address in my ipconfig /all result.(Physical address)
2) Second method would be to run a simple program, its called wireless network watcher. You can download it from this site.
You don’t need to install it, just run it and it will show the connected devices to your Wi-Fi..
In this case the first item is your router and second is my laptop. It’s a very good tool to have.