There's no proof of this. The only things that are seen are the outward acts. If the acts are positive or seen as beneficial to society, they're seen as altruistic. If they're negative or seen as detrimental to society, they're seen as psychopathic.
Psychopaths can learn how to act like they feel empathy for others and to say that they are acting on behalf of others. There's no real test to distinguish them from altruists except for what they say.
Altruists' motive might be to feel more self-righteous and put themselves above others, even if only in their own eyes. Psychopaths might have the same motives.