By using ion channeling and convergent beam electron diffraction techniques, we have determined the absolute polarity of various GaN films grown by MOCVD on (0001) sapphire. We observe two main classes of GaN films, namely flat and rough pyramidal ones. We find that flat GaN films have a Ga polarity. Rough pyramidal samples contain many tiny columnar inversion domains (with Ga polarity) imbedded in a matrix exhibiting an N polarity.