Q. What’s the simplest way to tell Rashi and Tosefos apart on a Talmud page?

A. Rashi is always on the inner margin closest to the book binding, Tosefos is always on the outside margin.

In 2007 we did a class on this, prepared and presented with then students Craig and Justin, titled “Seussian Talmud” to illustrate the difference in these Talmud commentator styles.