If they have a big, silly, open-mouthed grin, that’s already a good start. Often, their movements will be exaggerated: bouncy movements, and even exaggerated growling. Dogs may do the “play bow” when playing, where their front end is down, and back end is in the air. Sometimes, dogs trying to initiate play will slap their front legs down on the ground repeatedly. Although play-growling is exaggerated and friendly, it can be loud and can sound scary. Dogs who are play-growling are over-exaggerating their snarls, which may make this noise sound scarier than if they were actually fighting. If dogs voluntarily make themselves vulnerable by “falling” down, they’re probably playing. By exposing their bellies to another dog, they are being particularly vulnerable. Playing dogs will allow themselves to be caught while playing chase, and take turns chasing each other. Dogs who are playing will keep circling back from more. Even dogs that end up “losing” the play-fight and end up on their backs probably won’t want to stop playing. Dogs will take turns with most play-fighting behaviors.
Dog’s bodies when they’re upset or feel threatened by another dogs will become very stiff. Their hackles, which is the hair on their upper backs, are raised. If your dog has long hair, this may be harder to spot. Their growls will be different too. They will have low warning growls, closed mouths, and curled lips. Their ears will be pinned flat — no big, silly smiles. Dogs are trying to get away from one another, or at least one probably is, and their body language reflects that. They won’t be happy and bouncy. Instead, their tail may also be tucked underneath them. A dog that feels threatened will have quick and efficient movements. They won’t be bouncing around, and definitely won’t be “taking turns” like they did when they were playing. If dogs do end up fighting, it will hopefully be a short encounter. Unlike playing, the “loser” won’t stick around, and will try to leave the area. Since they’re not playing, they’re also not going back to one another for more play. Neither dog is likely having fun, but the loser certainly isn’t.