(this article is adapted from material I wrote for a Search and Rescue team. They asked for tips for reading dogs during evaluations.  This is also good advice if you are thinking of approaching or petting a dog.)

When you tell people your dog is friendly, it is best if you have words to describe "friendly". "Friendly" means different things to different people. If everyone uses the same definition, misunderstandings are minimized.

First of all, Friendly Dogs are not shy dogs and they are not pushy dogs. Dogs who are super active and/or try to take up your space upon greeting are not friendly, they are either anxious or pushy; sometimes a combination of both. It's the anxiety that makes them pushy...

Instead of "friendly" perhaps thinking of the word "neutral" is more helpful.

Neutral (Friendly) Dogs on Human Approach

• Relaxed and no body tension.  In the photo at right, both dogs show soft eyes, half-mast ears, and a relaxed jaw and lips.
• Mouth mostly open (think of my Border Collie – she's a real mouth breather when she approaches you. In humans it makes us look like idiots, but I love that idiotic look on a dog, because it means they won't bite me.) Or going from closed mouth to open mouth frequently.
• Lip Licking
• Eyes are blinking, the eyebrows are relaxed
• Can you reach over the dog and pick up a foot and have the dog relax within a few (5 or less) seconds? The dog should remain relaxed and totally resigned, relaxed and totally accepting or you might get greeting behaviours while the dog still allows you to lean over and pick up the foot.
• If the dog is blinking in an exaggerated slow way, they are being really sweet
• They are generally calm around people and the movements that people make
• They are not under your feet – this is actually a dog trying to take up your space
• In the photo at right, you see soft, squinty eyes, ears pulled back, and a relaxed head raised in "puppy-licking" type stance.
• They do not push on you when you pet them. A dog that is pushing on you could be affectionate IF IF IF the body is still very relaxed and you could actually sort of "pose the head" like turning the head on a Barbie doll.
• Ears half mast or down, but relaxed.
• Dog looks at you when you approach JUST LIKE she is looking at her owner. There might be a brief mouth close, then the mouth opens again pretty quick. 
• If you reach for the dog it does not duck away or, worse yet, freeze up 
• The dog's body is moving and soft, when you touch the dog there is no momentary stillness, or freezing or any sort of increase in body tension.
• There is not a big difference: increase or decrease in activity when you touch the dog
• Any tail wagging is low and "soft", sweeping in motion
• Looking right into your eyes with a soft eye, relaxed facial expression and mouth open or mouth closed with relaxed lips.

Dog to Dog

I am not so sure I like the word "friendly". In a working situation I would prefer a Neutral Dog – one that ignores others, and if a dog approaches, the dog may do a brief greeting or mostly ignore the other dog.

• CALM, relaxed – lack of body tension. Did I say that before? Like a million times? In the photo above, the Golden Retreiver has softy, "squinty" eyes, half-mast ears, and a softening "front-on" position (by head turning).
• Dogs are not looking, orienting toward or staring at other dogs. Staring is defined as any looking in a direction for more than a count of 1-2.
• Head is lowered, but the neck and body are RELAXED. Head lowered with targeting, and/tension is a signal for resource guarding.
• Ears, relaxed and half mast or against the head
• During dog to dog greeting butt sniffs are kept to quite a distance from the anal face and are not prolonged. They are quick and polite.
• If another dog is rude or pushy or approaches too quickly, a neutral, confident dog will not "lose his head" and over react. First he will try ignoring, neutrality and relaxed body posture. He might certainly get fed up with the asshole dog and then tell him what for, with a growl, tensing or air snap, but that would be second and it will even be done with a "calmness." Not in a hectic frantic, spinning way.
• Preferably, upon a dog approach, if the owner is anywhere near, the dog might check in or look at the owner to see what is up.

In the photo (above, right), the Golden Retreiver has wide-open eyes with pupil dilation. Body tension evident in both dogs. The Aussie on the right showing a cautious stance - she KNOWS this is not going well but is using extremely poor judgement. The Golden has "Fish Butt" lips (a fish has to keep his butt closed really tight so he doesn't fill up with water) indicating tension in the jaw.

In this photo (at left), you see hard eyes, ears pulled back, nose drawn up, tongue flick out of front of mouth and the body tension evident in planting of front legs.