A human is walking a dog in the park. The dog is leading, the human follows the dog around, cleaning up after the dog, and occasionally give a treat before the dog runs off again with the human following behind. If the aliens see all this from high above, who do you think the alien see as the master?
Most likely the human. If the alien's race has the brains to travel to earth, they could deduce that it's the humans that build the architecture our societies consist of—that we're the ones who built this urban world, and that the dog is a domestic animal. In addition, nearly always the dog would be on a leash. If this was also the case, well, a master would never wear one.
What if the park scene is the first thing they see?
Talking about first impression. :p
There are too many factors to consider.
I guess I always look into things too deeply. But if I consider it in the most simple way possible, I guess if the dog has a leash, the human will appear in charge. Else, the dog.
It would be hard to tell who is pulling the leash. Some masters used to have their slaves following them on a rope.
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).