> These apps are distributed binary, yes?

Like OS X apps, iPhone apps are distributed as bundles (directories) containing all the required files (http://en.wikipedia.org/wiki/Application_Bundle).

The info.plist file is a plain xml of key/value pairs of properties that tell the OS how to handle the app, which kind of files can be opened with it etc.

AFAIK, the application that kids use to crack iPhone apps adds a key/value pair to the info.plist which is needed to load the app on a jailbroken device. So, you just need to check for its presence. If the key is there, the app has been cracked.