If you read Paul M. Sammon’s Future Noir: The Making of Blade Runner (which is more or less the definitive literary work on the film), Ridley Scott states unambiguously that Deckard is and was always meant to be a replicant.
From pp 390:
We began this discussion with an examination of one of Blade Runner’s most controversial elements: the unicorn. I’d like to wind our talk up with its other most high-profile ingredient: the question of whether Deckard is or isn’t a replicant.
Well in preparing the storyline, it always seemed logical that Deckard should find out he was a replicant. It seemed proper that a replicant detective might begin to wonder whether at some point the police department hadn’t done precisely the same thing to him.
So I always felt the amusing irony about Harrison’s character would be that he was, in fact, a synthetic human. A narrative detail which would always be hidden, except from those audience members who paid attention and got it. But Tandem felt this idea was corny. I said, “I don’t think it’s corny. I think it’s logical. It’s part of the full circle of the initial idea. Ties it off with a certain elegance, in fact.” That’s why, at the end of Blade Runner, Deckard picks up that teeny piece of foil -
-the tinfoil unicorn origami-
-right, the unicorn, which visually links up with his previous vision of seeing a unicorn. Which tells us that the Eddie Olmos character A) has been to Deckard’s apartment, and B) is giving Deckard a full blast of his own paranoia. Gaff’s message there is “Listen, pal. I know your innermost thoughts. Therefore you’re a replicant. How else would I know this?”
…and with regards to other clues about Deckards replicant status…
pp391
-but I must say I better appreciate the more subtle suggestions that Decard might be a replicant. Such as the fact that collects photographs, which you see scattered over his piano. And of course the most significant visual clue is that over-the-shoulder, out-of-focus shot in Deckard’s kitchen, when you see Ford’s eyes briefly glowing. Was that setup intentional?
Totally intentional, sir. I was hoping there’d be those who’d pick up on that.
Since Blade Runner is a paranoid film, throughout there is this suggestion that Deckard may be a replicant himself. His glowing eyes were another allusion to that notion, another of the subtle little bits and pieces which were all leading up to that scene in the end where Deckard retrieves Gaff’s tinfoil unicorn and realizes the man knows his secret thoughts.
Actually, though, my chief purpose in having Deckard’s eyes glow was to prepare the audience for the moment when Ford nods after he picks up the unicorn. I had assumed that if I’d clued them in earlier, by showing Harrison Ford’s eyes glowing, some viewers might be thinking “Hey, maybe he’s a replicant, too.” Then when Deckard picked up the tinfoil unicorn and nodded - a signal that Ford is thinking, “Yes, I know why Gaff left this behind” - the same viewers would realize their suspicions had been confirmed.
I don’t think it gets any more definitive than that.