Stealing my internet?

Im pretty sure we're on 40gb per month
and that i''m the main user of it
apparently we have used up half of it with 2/3 of our month to go
Im pretty sure I haven't used up that much

I think someone may be using our wireless
How would i check?

If you are so limited to downloading, you might wanna check if you have any updaters or anything running.

Updaters are things that may eat even up to 10gb.
Also, just by playing online games for a month you can get 5gb off.

If you want to check out if anyones using your wireless, go to your ISP lan page
(in my case it's https://192.168.0.1)

If you go to your connections you can check out the incoming IP's.
You can also set up a wireless password protection (depending on your router/modem)

That's what passwords are for!