Click on this link for an ASPCA Dog Behavior Chart to help us understand when to intervene and when to let well enough alone as we monitor our dogs at play.  One of the most helpful suggestions on the chart, (inexplicably in very small print at the bottom!) is that when one is not sure if the dogs are playing or fighting, put the more aggressive one on leash and walk away.  If the other dog follows, it is likely that all is well–if not, probably the dog has had enough.