Yes, he is. You can either have Ridley Scott flat out say he is:
Deckard as Replicant or you can watch the movie.
The clues are right there. Watch the eyes of the characters. Every single replicant in the film, including the owl (but not every animal in the market as that would just be too obvious) has a shot where their eyes reflect red through the pupils. This includes Deckard. Now for the owl, that could have just been a natural occurrence while filming since owls are nocturnal. But why would each of the known replicants, plus Deckard have the same effect happen with their eyes. It never happens with Gaff, Bryant, Sebastien, or Tyrell. Plus, look at the severe physical beating Deckard takes throughout the movie. Leon slamming him into the truck would have easily killed him if he was human given Leon's exceptional strength. He then still manages to get his ass handed to him by Pris and Roy, but keeps going. I don't see any way that Deckard couldn't be a replicant.