I doubt they could find out by doing an MRI. MRI's are going to show structure of the brain and other things more and you really wont be able to see chemicals. The only thing they would be able to see is if the drugs effected your body in some drastic way which I doubt and even if they wouldnt really do anything if that isnt what their purpose was. They also wont spend money trying to figure it out cause there is a lot cheaper ways to do it.