The unicorn dream is obviously the strongest bit of evidence that Deckard is a replicant.
A unicorn is a symbol of a mythical (i.e. man-made) creature of beauty and purity - so it's heavily symbolic for Rachael. The dream is symbolic of Deckard falling in love with Rachael. Gaff's unicorn at the end is also symbolic of Rachael. Also, Gaff's other uses of origami/similar throughout the film are all heavily symbolic of the scene it's placed in - why would this one use be the only one not symbolic?
There's also the little hint when Rachael asks him if he's ever taken the VK test himself.
More a reference to the book, where the same question comes up, and he has.
When the police first hire him, he's told that SIX replicants hijacked the shuttle and one got fried running through a force field. He then gets info about Leon, Roy, Pris and Zora ... so where's #6?
Production error. They were originally going to have six (and even had the part cast), but had to cut one for budget reasons, and forgot to change it in the dialogue. This has been fixed in the Final Cut, as they changed the dialogue to having two replicants fried by Tyrel's security grid.
Deckard always seems to be physically out-classed by the replicants, which is evidence that he's not one of them, but he also takes a hell of a beating, which indicates that he might be.
I'd put this evidence more to him not being one. He even gets his ass kicked by Pris "a pleasure model", and while he takes a hell of a beating it's not past the realm of believability for humans. Also, he doesn't display the ability to ignore pain that the replicants do.
Gaff tells Deckard "You've done a man's job."
This is a colloquialism that means about the same as "you've done a good day's/hard day's work". It's not meant to imply that anything about the listener not being a man.
Also, every single writer on the project has said that Deckard was never meant to be a replicant. Plus, there are multiple literary themes that lose their impact if Deckard is a replicant, too.