select your target, and Get Info → build tab → architectures, click edit, and select from the available check boxes
you can also type in the text field for manual entry - specific processors can be listed, such as i686, ppc750 (for a G3), ppc7400 (for a G4) etc…
I assume that “ppc” will properly work on all PowerPC processors, from G3 up
make sure to do the same for the Project Info if you want future targets to be configured this way (I'm not sure if you could set this before you build any targets, and have the first target take the default project settings; either way, setting the target options always works)
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project info → general tab: in the Cross-Develop Using Target SDK pop-up menu, choose Mac OS X 10.4 (Universal)
(manual) make sure the project info → build tab → SDK Path is: /Developer/SDKs/MacOSX10.4u.sdk (or newer, if it exists)
Find out which architecture(s) a binary was compiled for
highlight the application icon, and use the Finder's “Get Info” command
in the terminal, navigate inside the application folder (.app) to Contents → MacOS, and run the following command:
lipo -info appname
you can also use the lipo tool to modify architectures in a package (remove unnecessary architectures to reduce application size, for example)