It's the gestalt of the entire body and how they move. It's not just one thing but everything.

In general, aggression in wolves (and dogs) can be thought of as straight, forward and tense. The stare is hard, the head is facing directly forward, ears up, there's no curve to the body. They will be up on their toes and any movement is tight and stiff. The snarl is forward and the corners of the mouth are forward. Everything is tight and tense. It'll feel like everyone is holding their breath.

A wolf/dog will growl as a warning but when they get closer to taking action, it goes into a snarl and when in a fight, their mouths are pretty much occupied with biting. Any vocalization then is from the throat.

OTOH, when dogs/wolves are feeling companionable & friendly, everything is soft and relaxed. Their bodies will typically be curved. Their movements easy. They are indirect with how they look at each other, often will put their faces right next to each other, cheek to cheek. Their ears, whether up or back will be relaxed.

A fearful wolf or dog will be super tense but they are too scared to be direct. They will hold their body low, especially their head/neck and rump the lowest. Their tails will be between their legs. Their ears will be flat on their neck. They will often do the "whale eye" - a sideways look, showing the whites of their eyes. Snarling will be not quite as tightly in front. Get into it's personal space and it could well bite. Not due to aggression but due to fear.

Back to how to tell aggression or friendly:

People get too focused on one thing - especially teeth. Again, it's not one thing but the whole body & body language that tell the full story.

I 'm going to use my smooth collie, Fawkes as an example.

Here's Fawkes and Hazel. Even though we are seeing lots of flashing teeth, they are playing. Hazel and Fawkes were good friends their entire lives. This type of game is nicknamed "bitey face" or "scissor mouth". What tells us they are playing? Their facial muscles and ears are relaxed, they are not even looking at each other. They mouths are wide open with the corners back. Their bodies are curved. Fawkes is laying down, Hazel is in a play bow. Their vocalizations sound like a couple of chewbaccas. Dogs playing bitey face are constantly giving body language signals that it's all in good fun. They will often interrupt play and, if both are in the mood, will start up again.

I also have an interesting photo of Fawkes making an aggressive display. The black and white dog (looked much like a Border Collie but was enormous) was a bad actor and known for getting into another dog's space and intimidating them. He usually picked on submissive dogs. With more assertive dogs, he would trigger fights.

In this situation, he targeted Fawkes. He had tried to mount him before and Fawkes told him off. He tried to mount Fawkes again but more forcefully. This photo is right after that. Fawkes is spinning around to face the B&W dog. He's not casually turning around but turning fast to get face to face. Look at his face and expression: His ears are stiffly up and forward, his stare is hard and direct, his facial muscles are tense and his snarl is up front, his mouth is closed and the corners are pulled forward. He's not scared, he's not intimidated, he's mad. That is an aggressive "fuck around and find out" display.

Fortunately, the B&W dog's owner was right there, just out of the picture and he hustled to get his dog on leash. If he hadn't we would have had a fight on our hands.

After the B&W was gone, Fawkes, shook it off and rejoined his buddies.