Each yes answer to one of the questions below increases the likelihood someone is lying to you:
Does the person have a history of lying?
Does the person have something to gain by lying to you? IE are they selling something?
Is the person a politician? Politicians almost always have something to gain by lying or exaggerating or not telling the whole truth.
Is the person in show biz or a celebrity? Like politicians, celebrities almost always have something to gain by hiding or shading the truth.