How do I determine the target architecture of static library (.a) on Mac OS X?

Another option is lipo; its output is brief and more readable than otool‘s.

An example:

% lipo -info /usr/lib/libiodbc.a 
Architectures in the fat file: /usr/lib/libiodbc.a are: x86_64 i386 ppc
% lipo -info libnonfatarchive.a
input file libnonfatarchive.a is not a fat file
Non-fat file: libnonfatarchive.a is architecture: i386
%