Ask a friend.

Other people — strangers, even — have an uncanny ability to detect when something's not right in someone else's relationship.

BYU psychologists tested this idea by having couples draw an object together, with one participant blindfolded and the other one giving instructions on what to draw. The whole thing was videotaped. Before they started, the scientists had the couples answer a few questions about their relationship in private, including whether or not they'd ever cheated.

Then, the researchers had a group of strangers watch the footage and guess which couples included a partner who'd ever cheated. The volunteers were surprisingly accurate.

Although preliminary, the research suggests that, simply by watching a couple doing something that requires working together, an outside observer may be able to detect infidelity or unhappiness.

"People make remarkably accurate judgments about others in a variety of situations after just a brief exposure to their behavior," the researchers wrote in the study.