Hugh, it was long time ago. Anyway: IR activity presupposes non-zero transition dipole moment, while Raman activity presupposes non-zero transition polarizability. Those modes which transform similarly to x, y or z (as shown by the character table) are IR active, while those, which transform similarly to xy, yz or xz are Raman active. See e.g. http://books.google.hu/books/about/Molecular_Vibrations.html?id=CPkvsDrPiv0C&redir_esc=y