It means that his basic directness and "honesty" are so strong that he can't lie convincingly. His true feelings are too obvious and his real meaning comes across even when he's lying. In this he is the opposite of the cold and calculating Michael Corleone, who is convincingly deceptive when he needs to be. Senor Sosa noticed this about Scarface when he said "I like you, Tony. There's no lying in you."