Dr. Edgemar claims it doesn't matter to him if he gets shot, because he isn't actually real. But when Quaid notices a drop of sweat running down Dr. Edgemar's face he takes it as a tell-tale sign that Dr. Edgemar is lying (why would he be afraid to die if he is not real?)
In the 2012 version:
Harry tells Quaid that Melina (and everything else) is not real. But when Quaid notices that Melina sheds a tear because she is afraid of losing the man she loves, he decides that she must be real.