Some folks argue that Deckard must be Human in the book because, at one point, he does actually take the Voigt-Kampff Empathy Test.
It is important to keep in mind that the one time in the book Deckard gets hooked up to the V-K machine, he believes he is human and isn’t trying to determine if he's an android. He simply asked officer Phil Resch to help him find out if he felt more empathy for female androids than other androids.
Deckard mentions (earlier in the book) that certain conditions must be met in order for the Voigt-Kampff test to determine whether the test subject is Human or Replicant.
(1) He tells Luba Luft, "one of the factors I'll record is the time-lag, if any. It's one of the factors we look for." Earlier he had told Rachael, “reaction time is a factor” yet tells Phil Resch during his own test merely to “watch the dials. We’ll exclude time lapse in this; I just want magnitude.”
(2) Earlier, Deckard told Luba Luft "it takes considerable experience” to administer the Voigt-Kampff properly. Phil Resch couldn't because he has zero experience with it.
(3) Finally, Deckard states that it takes “six or seven” questions to make a determination. When officer Phil Resch helps Deckard to test himself, Deckard only asked himself one question (in two parts).
So, the one time Deckard took the Voigt-Kampff test, it didn’t prove he’s Human.