heat sensors to detect flaming torches, and metal detectors to detect pitchforks
Pressure sensors inthe chairs / floor as people start to stand/ move more?
hand out sheets of cardboard with one side red and one side green. Then a simple light sensor could check for the amount of reflected red vs reflected green and come up with a rating based on that
The audience would be given the url as (for example) a projected QRcode, and once there, would be able to press a button on screen to register an active response (whether that's yes or no) or not respond
have you considered using a video camera and comparing each frame with the previous