I recently got notified that my data use was excessive on my cell plan. To the best of my recollection I was unaware of any changes in my habits so I was a bit perplexed as to why there would be an increased use.
My phone does not have the ability to determine how much data is being sucked by a specific app. Apparently there are some phones newer than my Droid X that can tell you that information. The Verizon rep suggested I turn off cellular data which can be found under settings and then battery and data management.
While there I discovered Data Saver. Turning on Data Saver restricts your application updating to when you are on WIFI therefore cutting your cellular data use. I turned that on and now I will wait to see what happens.
Another thing that will help is to make sure your applications DO NOT have automatic updating turned on. That seems to stress out the data plan as well. The more you can avoid unnecessary cellular use by using your WIFI will save you some money.
Good luck with getting your data use under control!