1st - Download this software from "Who Is On My Wifi" here
2nd - Then install the software on your PC and after installation, see image below
click the "Scan Page"
next click the "Scan Now"
Then that's it, you can determine now if there's someone who are connected on your wifi w/o your permission, those "Mac Address" with an orange color are the one who connect on your wifi that you didn't know but you can easily block them just check the "box" under the column "Block" but before you block you must be register first on "Who Is On My Wifi". But for my own style I just only use this software to know if there's someone connected on my wifi without my permission and if there's some connected w/o my permission, I just only change my wifi password.
That's all and hope it helps

