The easiest way is using the UI_USER_INTERFACE_IDIOM macro. Only SDKs 3.2 an later support it. Here an example:

if (UI_USER_INTERFACE_IDIOM()==UIUserInterfaceIdiomPad) 
  NSLog(@"I'm on iPad");

In the same way:

if (UI_USER_INTERFACE_IDIOM()==UIUserInterfaceIdiomPhone) {
  NSLog(@"I'm on iPhone or on an iPod touch"); 

The macro uses the [[UIDevice currentDevice] userInterfaceIdiom] method. If you are sure that your SDK supports the userInterfaceIdiom selector, you can also use something like this:

[[UIDevice currentDevice] userInterfaceIdiom]==UIUserInterfaceIdiomPad

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