(for what it's worth...)
the contractor that is excavating my project uses the following "field test" as a quick indicator of suitable clay. Take a hunk big enough to fit inside the palm of your hand, close your fingers around it and use your thumb and forefinger to work and squeeze out a flat ribbon about the width of your thumb. Continue to work the ribbon straight away from you until it breaks off. If you can get a strip about 3" long before it breaks, the initial indication is good clay soil.