Understanding how people perceive sincerity is important in identifying malevolent deceptive behavior in social media. This is a difficult problem because the only signal in most such cases is written text, and the larger proportion of human communication is non-verbal. Furthermore, perception is very subjective, and people adapt to each other's perceptions. We define the problem of decoding sincerity perception for two domains, dating and online games. We introduce and evaluate new psycholinguistic features of perceived sincerity. Our results shed light into the connection between language, deception, and perception, and underline the challenges and difficulty of assessing perceptions from written text.