In an interview with Ridley Scott he says that it is meant to be open to interpretation. Deckard's eyes glow towards the end of the film for a second, and when deckard walks out with his biatch right at the end and finds the unicorn, it is meant to confirm for those who saw his eyes glow that he is a replicant. however it is very open to interpretation.
Ridley in the interview says that Deckard may be a nexus 7 replicant. - A step closer to being actually human. He has human emotions, 'normal' strength, unlike the Nexus-6 replicants, and .... yeah.