As it was alread written in other answers,
otool -l yourlib.a | grep __LLVM
is the way to go.
An Apple engineer says using
otool -l yourlib.a | grep bitcode
is not reliable.
Searching for a “bitcode” section is not a reliable way to detect if your files contain embedded bitcode. If you want to do that, search for the “__LLVM” segment. You should be aware that a normal build with the -fembed-bitcode-marker option will produce minimal size embedded bitcode sections without any real content. This is done as a way of testing the bitcode-related aspects of your build without slowing down the build process. The actual bitcode content is included when you do an Archive build.
See also the comments by xCocoa.
It seems, that otool does not report the bitcode if code for the iPhone Simulator’s architecture is included (x86_64 or i386).
You can list the lib’s architectures with:
lipo -info yourlib.a
Then you can check for bitcode for each architecture separately, e.g:
otool -arch armv7 -l yourlib.a | grep bitcode
otool -arch arm64 -l yourlib.a | grep bitcode