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Spoiler!

Several characters within the Westworld control center are robots, not humans. Bernard Lowe, Theresa Cullen, and probably others. They were surprised to learn they were not human, or died without learning the truth about themselves.

These characters should learn very quickly they are not human. They would have different bodily functions than humans for food intake and defecation. Unless they were programmed to get tired, belch, sneeze, and cough like humans, they would have no need for those actions.

They would not get sick, but would see their coworkers take time off for the flu or a common cold. They would not age, but would see their coworkers slowly age over several years.

Memory works differently for humans than for robots. Human memory is vague, but robotic memory is so precise it is as though they relive the experience each time they remember a moment. They would be able to recall a memory from long ago exactly as it happened unless they told/programmed to "forget" it.

Humans should be able to remember key events from their childhoods and adolescence. Robots have neither a childhood nor an adolescence, so they should have no such memories unless several years of memories were implanted.

The point is they should have several clues which all point to the fact they are not like their human coworkers.

Shouldn't several characters have learned years ago they are robots?

I am only through the first season, so if this question is answered in a later episode, go ahead and mention that in an answer.

Robots can be programmed to ignore sensory input that they are not supposed to know.

Bernard shows Theresa a cottage in an obscure part of the park.

Theresa: This building isn't in any survey of the park.

Bernard: That's because we use hosts to do most of the surveys. They are programmed to ignore this place. They literally couldn't see this place if they were staring right at it.

Theresa: These unregistered hosts you told me about?

Bernard: He [Robert Ford] may have moved them.

Theresa: What's behind this door?

Bernard turns to look at door and then turns away from it.

Bernard: What door?

Theresa opens door and descends down stairway into lab.

Bernard looks at door perplexed and then enters.

Theresa: What is this place?

Bernard: It is a remote diagnostic facility. Ford and his partners used them when the park was in beta. I didn't know this one was here. The cottage must have been built on top of this place.

Theresa picks up several sketches of robots with faces.

Theresa: Have you seen these? What is this, Bernard?

Theresa shows sketches to Bernard. One of them looks like him.

Bernard: Doesn't look like anything to me.

Robert Ford enters the lab clearly aware of everything that Bernard and Theresa just discussed.

Ford: They cannot see the things that will hurt them. I have spared them that.

The fact that Bernard was literally looking right at a door just a few feet in front of him and did not see it was a major clue.

If hosts can be programmed to ignore sensory input, they can also be programmed to ignore bodily activities they don't conform to what their programmers want them to know.

Robots are programmed to "forget" any moment they experience which does not conform to their views of themselves as human. There are multiple scenes were robots are told to ignore conversations they had within the control center.

Robots might momentarily reflect on the fact they don't get sick at work while their coworkers do, and then discard that thought. They might go to a special toilet for robots and the moment they step outside that toilet, forget that they don't defecate as humans do.