IPad 2 / Retina iPad, how does the application determine which device it works on?

I'm trying to figure out how an application knows which version of Idevice it is included in. I know how to separate iPhone and iPad from a universal application, but what about the difference between the versions of Ie ipad2 devices, the retina of the iPad. What if I want to say the application on ipad2, and not the retina of the iPad, so as not to use the retina version of the game or to give the user a choice, which version they will pre-select