I think you are right. Unless there are very obvious signals that suggest a person is not truthful and our past experiences didn't teach us differently, we may be inclined to trust the words coming out from a stranger's mouth. Especially true while very young, thus the advice: "Don't talk to strangers!".
Liars who use this as a weapon are very good at dissimulating their intentions and acting naturally while lying to you. That's why this ability of ours to tell who's lying is not very reliable. But as I said, some people are better at it than others.