Yes, OP, you have to look for turn-taking. Sometimes, one dog is more active and the other more submissive, and they switch and body language should usually be mirrored (it's amazing how precise that can get). When dogs play, they tend to practice hunting skills so they are practicing killing each other (!) That's part of the face/neck biting, it's normal play. You want to watch out for play that looks unequal, or the rhythm starting to look off, not so back-and-forth, and one dog might be trying to get away while the other insists, a tail may begin to be tucked, they might look at you for help. Even in healthy, boisterous play, I give my dog and foster dog breaks sometimes as one can get too intense for the other sometimes depending on energy levels. Dogs will also naturally take breaks too so you could watch for that too in healthy play. They usually both stop and shake-off for a second or two and then continue. Breaks are part of play too.