$ cat rtti.cpp 
#include <iostream>
int main() {
#if !(__has_feature(cxx_rtti))
  std::cout << " NO RTTI " <<std::endl;
#else
  std::cout << " HAS RTTI " <<std::endl;
#endif
}

$ clang++ rtti.cpp -o rtti
$ ./rtti 
 HAS RTTI 


RTTI can be disabled by passing -fno-rtti.
 $ clang++ rtti.cpp -o rtti -fno-rtti
 $ ./rtti 
 NO RTTI

This was discovered when libc++ was enabled as the default c++ library.