What the hell is a "replicant"?
If you can't tell them from humans with medical/biological tests, such as on their DNA, then they are in fact human. Our DNA is what defines us as human in the first place, so if "replicant" DNA tests as human DNA then it is human DNA, and replicants are human by definition; not natural-born apparently, but human nonetheless. But if that's the case, what's with the talk about robots in the opening onscreen text?
Also, the idea that you could positively identify one simply by asking questions is absurd. There is no iron-clad set of parameters that human emotions/responses fall into, meaning there is no possible result to a question-based test that could conclusively establish that the test subject isn't human. It's the same idea as a polygraph test, which has about as much scientific credibility as a cold reading from Miss Cleo.