When the aliens see a human walking a dog in the park

I've heard this as a joke too many times I have to agree with Mantis, it would probably be obvious even at the first glance that the human is in charge. After all, the leash is around the dogs neck. If they look long enough, the owner will probably shout a few commands at the dog, which would also probably be a dead giveaway.
The only reason I can think of is if the alien culture is a lot different from ours, and there it's a sign of power being walked on a leash? (OK, now I'm clearly joking, but I can imagine a similar reason).