I know, I know, this sounds like a strange one. Most people know how to read basic body language. Like someone crossing his arms means he’s rejecting your idea. And cocking her head means she’s interested in what you’re saying.
But here’s a seldom recognized one to be on the lookout for (and avoid doing it yourself!) If someone touches his ear while you are talking, it is evocative of putting his hands over his ears to block out the sound. Subtle, but proven. To be safe, just keep your hands away from your ears when someone is talking to you!