This is an issue that I have looked at many a time myself - especially on those days where our internet usage has reached around 100GB for ONE DAY!
The best way to monitor what is going on with your home internet is through your router.
Some routers provide logs of internet usage by IP address, allowing you to track down the data gobbler in your house.
While this may be possible for some, it is certainly not possible with the modem/router that Telstra provided for our home internet service. It has really limited capabilities and cannot be configured to provide any monitoring capabilities. One of these days, I am going to buy another router to replace it so that I can have the extra features that I need.
Depending on how 'locked down' your own router is, you can also look at using something called OpenDNS to set up parental controls on a router that does not provide adequate features in this area.
Now I know this will sound technical, but I'll say it anyway! To access OpenDNS, you 'just' need to change your router’s DNS server settings to use OpenDNS instead of the default DNS settings defined by your internet provider.
If you are brave enough to look into this OpenDNS option further, click here to see how OpenDNS allows you to set up an account and configure web filtering.
Below are a couple of articles I have 'scooped' on this topic on the iTandCoffee Scoop.it page. If you need help to check out and configure your router or OpenDNS, call iTandCoffee on 1300 885 420.