Sketching the derivative of the function. There are also matching graphs and derivatives available at [url=https://www.univie.ac.at/moe/tests/diff1/ablerkennen.html]https://www.univie.ac.at/moe/tests/diff1/ablerkennen.html[/url] and [url]http://www.maa.org/sites/default/files/images/upload_library/47/Margolius/matchinggame.html[/url]