Ears

Because of variations among breeds' ears—some are pointy, others floppy—a dog's ears are a little harder to read, but in general, the same principles for cats apply. "When your dog is relaxed, comfortable and under no stress, her ears will be held in the natural position. When your dog is alert and watching something closely, her ears will be raised and turned to whatever she is paying attention to," says Dr. Burch. In addition, she says, if the ears are gently pulled back, it’s a “sign of a friendly greeting.” If the ears are completely pulled down and back, however, your pooch is most likely feeling afraid.

Eyes 

People use the term "puppy-dog eyes" for good reason: A dog's eyes can practically express the same emotional diversity as a human's. "When a dog is stressed or frightened, the eyes are not as wide open and they appear smaller. If the dog starts to squint (assuming there is no sun in his eyes), it could be a sign he is in pain," Dr. Burch says. Not only will a dog change the size and shape of its eyes, but the direction of its gaze is also a clear indicator of mood. Be forewarned that if a dog stares at you squarely in the eyes, or avoids looking at you in a way that lets the whites of his eyes show, he's on the defensive, so steer clear. 

Mouth 

A dog baring its teeth is a universal sign of aggression, but many people don’t know that a dog can also express other feelings with its mouth. "When your dog is relaxed, its mouth is usually closed or open just a small bit. Dogs who are stressed or afraid often close their mouth and the lips are pulled back at the corners," says Dr. Burch. To better understand your dog's mood, it helps to factor in its whole body, advises McMillan Loehr. "One interesting thing dogs do that can be mistaken for aggression is the submissive grin; that's when they pull their lips back from their teeth, which might make you think the dog is going to kill you," she says. "Put it in context with the rest of the body.” If her posture is relaxed and not stiff, “she might be doing a submissive grin." 

Tail

While a wagging tail generally does mean that a dog is happy or excited, that's not always the case, according to Dr. Burch. "One of the greatest myths regarding canine body language has to do with the dog’s tail. The myth is that a wagging tail is the sign of a friendly dog," she says. "A dog that is thinking about attacking may hold his tail high and move it back and forth. The key is to look at the rest of the body; if you see the wagging tail with stiff legs, tense muscles and lips that are starting to be pulled back, you could be in for some trouble." 

Body Posture

A dog's body posture tells a similar story to that of a cat. "Aggressive dogs try to make themselves look as big as possible. Their legs are stiff and they sometimes rise up on their toes. Dogs that are afraid may lower their bodies, dropping to the ground as if to say, 'It’s OK, I’m so tiny and small, I’m not a threat,'" says Dr. Burch. The one posture that people seem to misunderstand the most, says McMillan Loehr, is when dogs freeze. "A dog who is panting and then stops panting—that's a sign that something is about to happen. A freeze is a sign that a dog is getting uncomfortable and it is often a threat," she says. Signs a dog is happy include an open mouth that looks relaxed and a shift in weight from side to side. Another big cue, adds Dr. Halligan, is when their body is curved into a C shape, which is called a "play bow." 

Sounds 

If your dog’s movements aren’t telling you enough, listen to his bark. Barking can indicate any number of things. According to Dr. Burch, you have to take it in context. "Barking related to play will usually be accompanied with a relaxed body posture and sometimes a wagging tail, whereas barks that are short, insistent yips can mean 'Stop that!'" she says. "If you see a stiff body along with a low-pitched growl, the barking can be a warning sign; also, some dogs have barks that are intended to get your attention." And then there’s watchdog barking, which is a series of short and loud barks. "It’s a warning or alert bark designed to let you know someone is coming—and to let [whoever that is] know there is a dog here that is ready to handle the situation," Dr. Burch says.

Behavior

When all else fails, watch for significant behavioral changes. Destructive behavior, like chewing things, is a big cue your dog is not happy. "The number-one reason they're destructive is that they're not getting out enough or being exercised enough," Dr. Halligan says. Another cue that something's amiss about their mental state is a drastic change in behavior—sleeping in a different place, hiding more or sleeping more than usual.