How to monitor device with excessive wifi usage

My younger sister has an Amazon Kindle Fire 10", and according to xfinity it uses more data than the entire rest of the household combined. This morning I opened the xfi app, and found that this trend is continuing with 7 hours of network activity by noon, however, her tablet "freetime" doesn't allow her to use the tablet till four in the afternoon! What I am wondering is, what is the best way to scan/monitor/check for, foul play or a malfunctioning app? If Xfinity is to be believed, this "glitch" has used over 400 GB of network data in 10 days! (I am a power-user and am ready to use-and-abuse all my developer rights to figure this out)