On OS X, how do I find out what architecture a shared lib is compiled for?

just type file libjpeg.dylib and you’ll get output like the following

libpoll.dylib: Mach-O universal binary with 3 architectures
libpoll.dylib (for architecture x86_64): Mach-O 64-bit dynamically linked shared library x86_64
libpoll.dylib (for architecture i386): Mach-O dynamically linked shared library i386
libpoll.dylib (for architecture ppc7400): Mach-O dynamically linked shared library ppc