how can i find out who's stealing my wifi?

I doubt it's someone using your Wifi.

Even if someone managed to hack your password the first time, when someone changes the password, people get discouraged.

Since its home Wifi, it'd have to be a neighbor. And most people either A. Have internet in their homes or B. There's another house within their range with NO security that they'd use instead.

So it's more likely that there's just extra usage you're not accounting for.

Also note that whenever you go to check data usage, it's usually delayed (as in, when you check, that's what data was at a few hours ago)

Change your password once more. Mac filtering is a good idea. Hide your SSID as well. That will make sure only your household would know there's a wifi in the house. Nobody else can connect if they cannot detect the SSID.

Also, I don' believe there's any unauthorized user of your wifi as you have said you recognize all devices when you look at the router's list of connected devices.

you will not find out exactly who it is unless they used their name to name their pc. your router should allow mac filtering, allow all your devices in the mac filtering page, any other device trying to connect will now not be able to but i would have thought changing the password as you have done would stop anyone from connecting. it is possible you are using more data than you might have thought, streaming hd video uses a lot of data, even normal youtube videos can soon mount up the data usage

it is totally unlawful and ought to get you screwed over notably complicated yet once you extremely wanna know, there is a few utility obtainable that helps you to crack wifi passwords. purely youtube it.