An expert on human behaviour has revealed four ways that suggests if a person is lying.

Marc Salem, a men's health non-verbal behaviour expert, asks: Is your boyfriend staring a bit too hard into your eyes as you seek to find out about a lipstick stain on his shirt? Or does your fidgety fellow worker sit still when you seek to find out what happened to his favourite pen kept on his table? Salem was quoted as saying in British tabloid Daily Mail online that such unusual reactions could be more telling than either of their answers.

In addition to a steady gaze or displays of inconsistent behaviour, the other two give-aways get reflected through your smile or the lack of one.

Covering your mouth while talking or coughing an inordinate amount for a healthy person could also be ways to hide more than just one's smile, he said.

"A genuine smile changes a person's whole face," Salem noted, adding "Their eyes light up, and their cheeks and eyebrows rise along with the corners of their mouth."

"A fake smile appears in an instant, and disappears just as quickly."

He underlined that when a person tries to hide his or her mouth, it may be an inadvertent tell that he or she is hiding the truth.

On the flip side, if someone smiles too quickly or does so under unusual circumstances, the person may be trying to cover up a bad behaviour.