Slight edit: this should be able to be discerned visually- no blood tests, etc. we as humans can make this discernment both immediately and innately using visual cues only - can you figure out the answer?

In high school, I had an English teacher who would basically give you a hundred in his class. It was infinite extra credit if you could answer this thought experiment. Imagine that an alien species visited Earth and they wanted one of each animal for their Earth Zoo. In their visit, they acknowledged that humans are the dominant species, so to speak, of Earth. And so they want to exempt humans from their zoo. So they come to you with the question, "Can you make us a list of all the criteria that separates humans from other animals?" Something that we could read through and say, "All humans fit in category A, all non-humans fit in category B with no crossover." This should be easy because as humans, we can innately identify other humans. It's not an easy question though. Let me know if you have any ideas because this question still is a constant in the back of my mind almost 20 years later.