A few ways to tell are if they try to stand on other puppies/dogs. "Mating" with your legs are other items are a way of showing dominance. Its been a while I trying to remember all of the signs Layla gave us as a puppy that she is the alpha. Those where the main two. She started it when we got her and she was between 2 and 3 months, so I think that they will somehow show some indication. She is most dominate at home like your pup is though. I think you will be surprised on how they will take on their roles, even with the size difference. Tonya is right though, really watch them untill you know for sure there will be no problems.