Often? No, you can't. By and large you can make a reasonable guess as to what will piss someone off; but this doesn't mean we understand anger. We understand that gravity is a force that acts between two objects, for instance, but we don't understand what gravity itself is at all. But this does not mean we must presume that gravity does not exist; likewise, we can see the impacts of emotions, and observe their effects, even if we don't really understand what they are on an intellectual level.