This is a photo of two of my dogs playing. They sound like dinosaurs from Jurassic Park. Patrick (the little blond one) looks and acts as if he is going to kill Anton. He makes a ferocious "mean face" and you do not want to challenge him when he has a tennis ball!
How do I know that this is play?
Anton is not bothered. He continues to engage with Patrick. He is not giving calming signals (lip licking, lifting a paw, looking away), or leaving the room.
There is no blood. Nobody gets hurt.
Between attacks the dogs stop, pant, and wag their tails at each other.
They look scary. They sound scary (mostly Patrick).
Sometimes when they play I feel my body becoming alarmed at the savagery of their engagement, and I have to remind myself that this is how dogs play. Playing this hard and with this intense level of activity and interaction is important for their mental health.
Studying resources like Brenda Aloff's Canine Body Language and Turid Rugaas' Calming Signals helped teach me the difference between battle and play. I read those books when I had two terriers who were trying to kill each other (well, at least Ariel was trying to kill Tootsie). These are necessities for any dog owner, and particularly for people who rescue, people with multiple dogs, and people whose dog ever has to interact with another dog (say, passing another dog while out for a walk).
The value of playing this hard wtihout reserve reduces their frustration at having to live in a home with humans, adhering to human rules. It's just like when we hang out with our best friends, when we can totally be ourselves without moderating our own behavior. We like other people better after that kind of relaxation.