Sometimes the simplest things will slip by you for the longest time.
After time after time of trying to find a definitive list of macros available for use in RPM specfiles on the Internet, I finally read the rpmbuild man page just to find the ‘–showrc’ option. From the docs:
The command rpmbuild –showrc shows the values rpmbuild will use for all of the options are currently set in rpmrc and macros configuration file(s).
Now I can easily run rpmbuild –showrc and grep the output for whatever I’m looking for.