Detecting the state of an in app purchase

How can an app check the state of an in app purchase and see whether is has been purchased or not?

I want to do this in the viewWillAppear and set the icons of the in app purchases according to w开发者_如何学运维hether they are purchased or not.

Can this be done with the apple libraries?

This is a great tutorial which shows you all the steps in setting up in app purchases, including sample code on how to do it. Hope that Helps!