There is no evidence in the novel. The film is another matter. Ridley Scott has said he intended Deckard to be a replicant, even though the screenwriters were unaware of this idea and disagree. Hence, the famous debate.

1) His eyes go red (as all the replicants eyes do) in one shot. 2) In some versions, there appears to be a continuity error on the script, which mentions six replicants escaped. Is Deckard the final one of those six, reprogrammed to capture the others?

3) He never replies to the question about taking the Voight-Kampff test himself. 4) Most compellingly, he dreams of a unicorn, and later finds the origami unicorn left by Gaff. If we assume he told no one about the dream (including Gaff), this can't be a coincidence. Is Gaff letting him know the truth, ie that the dream was programmed and he is a replicant? Is he also implying that he and Rachel are being allowed to leave as a sort of controlled experiment?

Ultimately, the question of whether Deckard is a replicant is left to the viewer, but the ambiguity is what gives the film its strength. I don't think it should be definitively answered, but there is definitely evidence he is a replicant. :)