Conveying vs. Convincing
Without knowing specific facts before a person speaks, it is nearly impossible to claim you know a person is lying. A person who is well trained in body language, psychology, and general deception will tell you a person is displaying signs of deception. They know there are many signs of deception, but those signs can also be general stress.
A person may exhibit stress while lying, but they may also exhibit stress due the the nature of a question being asked or a topic being conveyed.
The chart to the right displays the natural cognitive progression of truth and deception. Expressing details that are true are normally much easier (depending on the topic) than attempting to deceive another person. As instructed in "Paralanguage", the tax on the brain and the resulting observable signs are what allow the investigator to determine deception or truth.