If you want to know the architectures a library supports, just call:
lipo -info <libname>
lipo is also used to create fat-binaries.
If you want to look more deeply into a binary library the easiest way to use nm.
NM will list the whole structure of the library together with the public classes/method.
It’s under the Xcode.app bundle.
For example:
/Applications/Xcode.app/Contents/Developer/usr/bin/nm -g -U -A -m -arch armv7
The tools can be found at /Applications/XCode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin