Most educated dog trainers stay away from labeling a dog aggressive. Generally, I never use the word in my practice, as it's not a particularly helpful term. But lots of owners use the term, and often they use it incorrectly, so I thought it was worth devoting a blog to it.
I get calls about aggressive dogs and people describe their dog as playing aggressively, yet most of the time, I see a playful dog who is just lacking in self-control.
What's the difference, and how can you tell? Ask yourself: is this dog trying to hurt me (the other person, the other dog)? Does it seem like he'd be happier if I (or the other dog) went away?
A dog that plays too roughly just needs some social skills, but isn't really aggressive. If he means to do you or the other dog harm, if he leaves punctures, well, then maybe we're now leaning toward aggression. The label itself isn't particularly helpful (and it actually gets in the way of treating the problem). Instead of labeling a dog, an educated trainer will look at the actual behavior the dog is displaying.
Which brings me back to those first couple of questions: Is the dog trying to hurt me and increase distance. A lot of dogs look aggressive because they're scared. All they really want to do is make you move further away, they need more distance. So they growl, bark, snarl, or lift their lip. A smart person will move away from the dog.
Again, think about other sources of information-- is your dog uncomfortable with you (or the other dog) or is he just exhibiting over-the-top play? Is he stiff? Is he curling his lip? Generally, aggressive dogs will become stiff, they don't move very much (and when they do, they don't have soft curves, but rather their movements are sharp), they'll stare you down with hard, unfriendly eyes, and don't wag their tail in response to your calling their name.
An over-the-top dog looks goofy -- their tongue is sometimes lolling out of their mouth, their jumping, they may be mouthy, they're moving a lot and usually seeking interaction with you.
In any event, if you think your dog is aggressive, call a positive reinforcement trainer to get an expert opinion. It's not worth risking a bite "just to see" if your dog is over-the-top or is exhibiting dangerous behaviors.