Everyone knows someone who just might be a real psychopath, but how can you be certain? A recent study puts one more tool into your detection arsenal: psychopaths are much less likely to yawn around others who have yawned, basically because they're lacking in the empathy that makes the yawn contagion mechanism work. So while it's not exactly conclusive, that troublesome coworker's failure to yawn around the water cooler before an early-morning meeting might actually mean something after all.
A team of psychologists from Baylor University in Texas used experimental participants (both male and female) given a psychopathy personality test to investigate how yawning is related to empathy. The participants were then exposed to a "paradigm intended to produce a reactionary yawn" (i.e. videos of other people yawning). The psychologists' findings, recently published in the journal Personality and Individual Differences, were that the trait of "coldheartedness" (related to psychopathy) predicted a failure to yawn when shown the clips of others yawning. Though the study was rather small, with only a sample size of 135 participants, and though the correlations between coldheartedness, psychopathy, and reactionary yawning aren't perfect, it does suggest interesting avenues for further exploration of how empathy does (or doesn't) work.
At the end of the day, despite the inevitable frictions, humans are a really social species and the vast majority of humans are closely attuned to the feelings of others without even necessarily realizing it. (Heck, even dogs are emotionally attuned enough to humans that yawns become contagious between dogs and their owners). So if you meet someone who shows other symptoms of psychopathy, like superficially charming and sweet people who have chosen management professions and who take a ton of selfies, run don't walk if they fail to yawn when you do. Since psychopaths find it easy to blend in with normal people (unlike sociopaths, who act out), we'll take all the clues for identifying them that we can get.