Your pooch is licking his lips and yawning.
Yes, he might be hungry and, yes, he might be tired. But these signals can indicate a feeling of distress, as well. If you’re petting your dog and he yawns more than once, chances are he’d like some space. Give it to him.

Your pooch is bowing.
You might think he’s just stretching, but consider this a clear invitation to play. And when he’s also wagging his tail and has something like a toy monkey in his mouth, he might even be hard to ignore. Come get me!

Your pooch looks away when you look at him.
Not making eye contact is a sign of submission or deference. If you’re trying to get your muddy canine to hop into the tub for a bath, or to drop a stick, he is looking away to tell you, “I’m sorry, but I don’t want to do that.” With all due respect, of course.

Your pooch’s ears are tucked back.
If done while running, it’s for speed. But if his ears are in this position around another dog or a person, he is afraid or anxious. Respect his fear and remove him from the situation immediately.

Your pooch’s tail is straight up, perhaps wagging stiffly.
A wagging tail does not always equal a happy pup. If his tail is erect and vibrating slightly at the tip, stay alert: he feels tense or over stimulated. He might bark or lurch at another dog, or even at a nearby person, so keep him on a tight leash.

Your pooch’s lips are back, his mouth is relaxed and open, and his teeth are exposed.
No need to go Freudian here - sometimes a smile is just a smile. If your pooch’s tail is wagging at the same time, well, the happy guy might even be laughing at you. Dogs have a great sense of humor.

From AARP Magazine 2008