Measuring Consciousness

How can you tell if a creature is conscious? You can tell if it is not conscious if it does not react to stimulus, but even a plant reacts to stimulus (albeit slowly). You know that you are conscious and presumably other humans are the same, but everything else, you are just guessing until we find some way to measure or detect consciousness. What bothers me is so many scientists claim such and such a creature is or is not conscious. How could they possibly tell?

~ Roedy (1948-02-04 age:70)