data usage

Like many people here - my usage has gone up from - 400 - 600 - 700 - and now 1282 - for the month of December - - however my Family and I were GONE for 1 week?  So increased usage - and no one home??  trying to get details on which device - and Xfinity is like having a root canal - zero help - 

and chatting with them - please look at upgrading to unlimited...

bait and switch?  I just want to determine which device is causing the usage - it's a simple network tracing situation