Is it legal to test someone without their knowledge or consent? Like if I MITM the public toilet and start drug testing random passersby... well the results would probably be skewed due to contamination... Now that I think about it, this kind of massive, drive-by poo-veillance is actually a really hard problem.

Wait I'm getting distracted. Yeah, original question: is consent needed before such testing can be done? Mileage varies by state maybe?