The replicant leader, Clovis, is always a replicant.
Spencer Gregorian is always human.
Bullet Bob usually turns out to be human, but can actually be identified as a replicant as well. This can partially be a result of not calibrating your V-K properly but mainly indicates what a flaky character he is. If the V-K test shows him to be a replicant, then you are only doing your job if you retire him…
The others, like Lucy, Gordo, Izo, Sadik and Dektora, may or may not be replicants, depending on your game. This is randomly determined when you start a new game.