how to detect occupancy from images?

I have two images of same scene with me and what i want is to check for occupancy(whether the scene is occupied with any people). Count or specific person is not important for me.Just that a person is there or not. I need answers to these - I tried using template matching with SAD and also MaxAD.But i couldn't find anything useful(1). - Face,nose,mouth detection all of these are helpful but it even detects other than human features(2). - tried using imsubstract but how could i reach a conclusion of the presence of an human being so that i can use an option of (2)and (3).