Within iOS 7, there's a neat new feature to tell you which apps are chewing up your cellular data. If you visit Settings> Cellular and then scroll down, you'll see the awesomeness.
Here you have the ability to see how much total data you've used since the stats were reset, along with which apps are using it and how much they've consumed. You can also toggle off any apps you don't want to have access to cellular data if you're trying to conserve.
If you're interested in tracking that information for a specific period of time (a billing cycle, say), you can also scroll to the very bottom and pick Reset Statistics.
The only caveat here is that if you turn off cellular data for an app, it's really, truly off. So if you try to browse the App Store, for example, when you're not on Wi-Fi, it'll give you an unhappy warning.
And if you've forgotten what you've done, you may be very confused as to why you can no longer access the App Store outside of your house. So either be careful what you turn off, remember the changes you've made, or be like me and do a bunch of Google searching to troubleshoot the problem in a month or so. Haven't you all learned to never be like me?