How to tell how much Internet usage I have left on my AT&T iphone?

I have an iphone 5 and I have I think a 3gb plan it's a 4g phone and I want to know how much usage is left maybe how much I used, is there an easy way to check that? Like maybe an app or a website or on my phone would be great

Maybe on AT&T's website. Not sure, not a customer of theirs.

There's an AT&T app, that may be able to or if not there are many data monitoring apps which could help (in future).