Is there a way to see which iPhone an app is running on?

If there is what is the best way to do it?

Yes I believe it’s under the iPhone class.

iPhoneSettings.generation

But I just use Screen.width/height to check, so I can see the correct GUI-layout in Unity for 3GS/iP4+ and iPad.

iPhoneSettings doesn’t have any members anymore (according to the docs):
http://unity3d.com/support/documentation/ScriptReference/iPhoneSettings.html

But the iPhone class has several:
http://unity3d.com/support/documentation/ScriptReference/iPhone.html