Well usually if my dogs are standing real close to each other, and wagging their tails kind of slowly, looking identical, somebody gets called away. Some dogs get stiff, some go on their tippy toes, my bitches like to stand over other dogs or put their heads over other dogs shoulders, that's rude as hell. Sounds you really have to know your dog, I can tell when things are getting iffy, but most people think my dogs playing nicely is pretty vicious. Also some dogs are just socially dumb, like my Ripley who thinks throwing 60 pounds of herself straight into another is great fun, or jumping on/over them so I do a lot of interference so nobody eats her face, she doesn't take a dog correction very well.