You should be able to login to your router and go to the DHCP table or something similar that shows connections. If you feel like users are connected to your wifi that you don't want connected, change your Wifi password to something like [email protected]$$w0rd001! to prevent unauthorized users from gaining access.
[email protected]$$w0rd001! is a terrible password.
The only way to really prevent anyone from cracking your password is to use very long phrases with 30+ characters in it or, better yet, 30+ characters that are completely random and combine numbers, special characters, capitals and lower case letters.
You're right [email protected]$$w0rd001! is a terrible password as far as cyber security goes. I should've clarified that point. When protecting WiFi, it's not likely that a war driver would spend a ton of time on any given network trying to hack the password, especially if it's residential. So the sample password I suggested would easily protect the users WiFi from anyone making random guesses trying to log in.