How can I detect an error when searching through a property list?

Okay, here's the gist of my app:

On the first screen, you enter a 4 digit code. When you press done, it saves the code automatically switches views to the next screen. On the new screen, it pulls up the saved code and searches through a plist trying to find the string it's associated with. It currently works perfectly except for when the user enters a code that is not in the plist.

How can I teach it to give an error alert of the code is not present in the plist?

Thanks in advance!