Xcode Preprocessor Output
You can choose the "Show the Assistant editor" view in the top right set of buttons in the toolbar.
Then, at the top of the right-hand editor you can click the little Related Files button and then either choose Preprocess or Assembly.
You can see an example of both the preprocessor output and generated assembly code all side-by-side here:
Xcode 4.1 features now "Preprocessor or Assembly Output". All be happy!
@whoKnows's excellent and concise answer applies to Xcode 6 users. The preprocessor menu seems to wander all over the place from one Xcode version to another.
To recap: XCode 6 users can view preprocessor output by pulling down this menu:
Product -> Perform Action -> Preprocessas illustrated in the accompanying screenshot. I couldn't find this anywhere in the Xcode documentation.
(I hope this helps. I would have preferred to simply add a comment to @whoKnows's answer, but I don't have privileges for that yet.)
