Granted it would only work upon a gradual onset of symptoms in which on first sign of anomalous behavior the test is taken, because as you said, it wouldn't work on someone already influenced.

It's still better than nothing.

[He had to do something. What of House didn't warn him? What if he'd been affected to? What if he'd been made to kill--

--no. He couldn't go down that path of thought right now.]

I'm going to fashion a sort of Turing Test--except for use on people rather than computers to see if they're being influenced--questionnaire and distribute it, at the very least it could help in some situations.