Some people haven't knowledge about dog body language, warning signs and natural dog behaviour, which can cause misknowledge between owners (or between us - during the training - and other people in the park).
Make sure before you are labelling a dog as "aggressive" to have enough knowledge about their natural behaviour.
If the professional tell you the dog's behaviour is normal, please trust them and do not start to argue with them - especially if you don't know anything/too much about the topic of the argument.
I saw lots of surgeries during my life (on the TV and in real life) but never told the doctors how they should do their work because they should know better than me.
Dogs play by chasing, tackling and nipping at each other.
Signs of playing behaviour:
the play bow
big open-mouthed grin
relaxed body language
loud, continuous growling and snarling - (if the person is not an experienced owner or doesn't have too much knowledge about the dog's behaviour, this point can confuse and lead to label a dog as aggressive)
the dog keeps going back for more play
Signs of aggressive behaviour:
stiff movement, no bounce
the hair on the dog's upper back is raised
closed mouth, curled lip, warning growl
ears pinned flat
the dog will get away from the situation (who is the loser of the situation)
How can you prevent the fight (rough play)?
If you notice play is starting to be uncomfortable for a dog and be rough, separate the dogs for a few minutes to be calm down. Teach a good recall for your dog to respond in any environment. Try to keep toys and food out of the playing area because some dogs possess their items.
The lists are above is valid for adult dogs and puppies, too. We have lots of clients who tell us their puppy is aggressive, and 95% of these puppies have normal dog behaviour, but because of the missing of their owner's knowledge, they already have an "aggressive" label in their early life. Puppy aggression is uncommon for up to 12 weeks (if the puppy comes from a good place/breeder/owner and is separated at the right time from the mother and siblings), the missing of socialistaion can lead the dog to be aggressive later.