I believe a pencil test is done be taking your soil and rolling it with the palm of your hand into the shape of a pencil
If it holds shape you have good clay content

Other people say you can take a wet ball of soil and throw it at you truck door
If it sticks you have good clay
Make sure it's a pickup truck preferably with rust and duct tape holding something together
I believe that's called
"Redneck engineering "


Just remember you can't do that with beach sand