Context tells a great deal, and it is probably the easiest measure of dog interactions if you are not familiar with the finer details of dog body language. If you have two dogs who are friendly with each other generally and they start rearing and mouthing, they are just wrestling. If you have two dogs who are not familiar with each other and/or there is food or a toy involved, and there was growling and posturing involved before contact, then they are probably fighting.
Finer details to look for are the tail, as mentioned, but not whether it is wagging or not. Instead, look for the placement of the tail and the speed of the wag. If there is a high, over-the-back posture of the tail and it is wagging really really fast, that's NOT a good sign. A playful dog's tail will be at a mid-level (not over the back, not tucked under the body), and the wag will be intermittent or average.
Another detail to watch is the dog's face. Is it snarling? Are its lips relaxed or tense? A playful dog will be relaxed, whereas a dog in a fight is tense.
If *your* dogs are wrestling (don't try to break up a dog fight that involves dogs you don't know or dogs that are obviously biting and tearing at each other), but you are thinking it could be too much, here's a quick test: pull the dogs apart and watch the dog you thought was being bullied. If the dog goes back over to the other dog, they're just having fun.
Cedar (tri), Willow (blen), & Holly (ruby)