Back in the days before antibiotics, syphilis was a dreadful problem encountered on occasion by hapless sailors on shore leave bewitched by young maidens.
Fortunately they could use their super-secret special handshake to detect syphilis. A demonstration is below, feel free to use it when dating.
Amazing! And you can sneak it in when dancing if you miss your opportunity for the greeting handshake. Thanks to Mr. Science Show, the man in the video for the hat tip.