[alaskayorkie]

I found a Web site that has some good explanations of dog interaction. It's all interesting, but scroll down to Play and Play Invitations for the part you're interested in.

http://www.wagntrain.com/BodyLanguage.htm

From personal experience, you should be able to tell the difference when it gets out of hand. Both dogs are showing their gums, their teeth and the whites of the eyes, and you can tell injury is possible.

If there's no danger of injury, I let it go. If it seems to be getting out of hand, break it up with no punishment. Just sort of a rest period.