On the command line, gcc -E foo.m will show you the preprocessed output (just as it does for normal C/C++ files). Of course, this will also expand any #include or #import statements you may have in your code.
From within Xcode:
- Xcode 3: Select the file, then Build → Preprocess.
- Xcode 4: Select the file, then Product → Generate Output → Generate Preprocessed File.
- Xcode 5: Product → perform action → Preprocessed File.