tool for discovering posix capabilities

Is there a tool or some source I could compile that would tell me all
the posix features on that particular system?

As I understand it, a system might have compliance to one of three
different posix versions and even then maybe only partial compliance. I
already know that my system has some posix 1995 features, like pthreads,
but there is a long list of posix features I am not sure about. Rather
than using trial and error, I'd prefer a tool that would tell me what is
available so I can go off and read about it.