You can read your Info.plist as a dictionary with
[[NSBundle mainBundle] infoDictionary]
And you can easily get the supported orientations at the UISupportedInterfaceOrientations
key that way.
NSDictionary* infoDict = [[NSBundle mainBundle] infoDictionary];
NSArray* supportedOrientations = [infoDict objectForKey:@"UISupportedInterfaceOrientations"];