There are utilities you can install and get some results and Apple computers has one in their network tools. Since you are using that much data, you can disconnect all of your devices, change the SSID and password, connect a few devices each day, and check the usage each day which you can check by signing on to the Comcast web site. Things that may be causing an issue are things like back-ups to the cloud (Amazon Drive or Google Drive), a DDNS tool, a weather station that saves data to the web, cameras that are viewed remotely, a TV that uses the web like Samsung, a streaming device such as the netfix, or amazon stick, a gaming system, camera system that save data remotely,.. I had a Windows 10 laptop that every time I started it up it would try to load an update for 30+ minutes then fail, allowed me to work but if I shut it down, it would try to reload the update again and fail. So pay special attention to any device that is not performing correctly.

@Jimmystartup 

 

You are not able to monitor device usage. What you can do is to organize your QoS so low priority devices will not be able to use up to X amount of bandwidth.

One other thing to consider is you may have a device that has been hacked.