Face tackling would be called if a player LEADS with the face into a player. If his face happens to contact the player, that's nothing. What's the difference? While not stated, basically it comes down to intent. Did the tackler intend to create contact with his face mask (which hurts the player being tackled) or was he making a heads up tackle and his face mask just happened to make contact. It is a rare event.
Much more common is using the crown of the helmet to make contact, again (unstated) with the intent of using the helmet as a weapon. It can be done directly (spearing) or indirectly (running your helmet up into someone's chin).
In reality, it doesn't matter. All of them are the same foul: Illegal Helmet Contact. Did the player use his helmet as a weapon against his opponent? Even in making a legal tackle, helmets are sometimes going to hit first. Simple way of telling if he intended to use his helmet as a weapon: would he have made the same move if he wasn't wearing a helmet? If yes, he wasn't using it as a weapon. If no, he probably was.
