iOS 6 added the ability to see how much data you consume on a monthly basis, and iOS 7 added the ability to track how much cellular data you are using per app. Here's how to check how much cellular data you're using on both your iPad and iPhone.
1. Navigate to Settings
2. Tap the "Cellular" tab
3. Scroll down to "Call Time" to see your current period usage
4. Scroll down for an alphabetical list of all the apps you allow to use cellular data
5. Tap "System Services" for a breakdown of how much data each app such as Messaging Services and iTunes uses
You can also tap the blue "Reset Statistics" tab on the Cellular settings page to reset your counter. This is a good way to have your data counter coincide with your monthly cellular bill.