There are a lot of "hints"/indicators that suggest someone is lying, but I'm not sure anyone can ever be certain that someone else is not lying. People who are "good liars" often know about all those indicators, and can be pretty skilled at using them. Some of those "hints" come from the fact that the person feels bad about lying. Sociopaths don't feel bad about lying; so that, alone, gives them an an edge. I think in some cases it might help to ask a few more questions, because sometimes a person can be skilled at actually telling the lie but may mess up when required to provide additional information (and still control those indicators).
Sometimes we know that one person or another would absolutely never lie about one thing or another. Other times, I think, maybe, we have to accept that we can't be certain if someone is lying or not - at least not at the time someone has said something.