If you download the latest version of Xcode from the Mac app store, the simulator now includes the ability to simulate the Retina iPad display - in the simulator menu "Hardware", "Device", "iPad (Retina)".
Please note: since the display is so large, you can reduce the display scale of the simulator - in the menu, "Window", "Zoom", "25%".
This will allow you to make sure that if you download images with a resolution of 2X, they load correctly. This will not help you ensure the smoothness of your animations - for this you will need a real device. I tested my application with a simulator, and everything seemed fine, but when I understood my device, I had some problems with the increased resolution.
If you encoded your application in a resolution-independent manner and had an iPhone Retina, it would not hurt to run it there - this is a completely different context, but has some common features.
Ultimately, you know that your application correctly supports the Retina iPad display by testing it on a third-generation iPad.