How do I know if they're just playing?

Yes and no. How is that for a pat answer. Yes, sometimes Sam is just trying to get DeeDee to play, sometimes he is trying make DeeDee understand you belong to him and DeeDee is a third wheel. To get it to stop? Gee Whiz, folks have been asking that question for many years. It probably won;t stop to be honest. All you can do it keep watching out for DeeDee to become fatigued, and stop the interaction when it becomes too much for him.