Surely the only person who will ever know if a character is 'out of character' is the person who invented said character? They're the only one who truely knows what the character has been through in his life, what he is prone to acting like, what he is capable of acting like in certain situations etc.
Im a writer, and I don't like to think that my characters would suddenly be considered 'out of character' or 'wrong' if they did something surprising. Maybe it's just a side of the character you haven't seen before, or a side which was kept repressed, or even a side which was there but not written about until now?