Yes, but there are things that an LLM architecturally just can't do, and LLM-specific failure modes, that would still give it away, even if being instructed to be deceptive would make it a bit harder.

Obviously as time goes on, and chatbots/AI progress then it'll become harder and harder to distinguish. Eventually we'll have AGI and AGI+ - capable of everything that we can do, including things such as emotional responses, but it'll still be detectable as an alien unless we get to the point of actually emulating a human being in considerable detail as opposed to building an artificial brain with most or all of the same functionality (if not the flavor).