You can use
#ifdef DEBUG
....
#endif
You’ll need to add DEBUG=1 to the project’s preprocessor symbol definitions in the Debug configuration’s settings as that’s not done for you automatically by Xcode.
I personally prefer doing DEBUG=1 over checking for NDEBUG=0, since the latter implies that the default build configuration is with debug information which you then have to explicitly turn off, whereas ‘DEBUG=1’ implies turning on debug only code.