Your built-in data meter should be enough. If you want something more specific, apps like Onavo (in a bit of hot water recently since they're owned by Facebook) or My Data Manager could work.
As @SpookDroid notes - there's one built right into the OS. I actually used it earlier this week to track down that Google Sheets, Slides and Docs were using data in the background even though I never use the apps themselves. From the list of apps using data, I was able to tap the app and disable background data.