How dogs sense fear I heard that dogs can hear the heartbeat of someone when they are in fear. Although a person's heartbeat is somewhat louder when in fear, it is also louder when exercising. I have never seen anything about dogs being able to hear the heartbeat. That is not to say they can't, but I would think other noises would drown out that sound. When people are in a state of fear, they give off odors in their sweat.
Dogs have extremely sensitive smelling ability and can probably pick up that odor.
Also, dogs can see the body language of a person in fear. Fooling a dog by moving like you are afraid can cause some dogs to attack you. I had an experience with sensing fear about 8 years ago. I can remember this like it happened yesterday. I was in my basement of my parents house watching TV, sitting on an old green chair.