In Xcode 6 GM run your app with the iPhone 6 or iPhone 6 Plus in the simulator and then hit cmd-s. This will save a screenshot to your computer.
iPhone 6 / iPhone 6 Plus as seen in Xcode 6 GM:
How to save a screenshot in the simulator:
EDIT:
Make sure Window => Scale is set to 100% in the simulator, else you have the correct device, but end up with the wrong dimensions in the screenshot. – Jos
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