Android Tracking which apps use your data is pretty simple. Just install My Data Manager (Free) and allow it to track the data your apps use.

Preventing unnecessary apps from running in the background helps avoid this issue. Finally, reducing background processes limits unwanted notifications and decreases unnecessary data usage.

Here you can disable 4G, if your network supports it, and turn off mobile data roaming when abroad. On Android, go to Settings>Data Usage and slide Mobile data to off.