In 2010, Pat Kirwin wrote the book Take Your Eye Off the Ball: How to Watch Football by Knowing Where to Look. In 2015, he put out version 2.0. For years while watching a football game I would watch the offensive line. If a guard pulled I could predict it would be a run, or if a tackle’s helmet would pop up before he backed into protection I could predict pass before the quarterback took his second step.
Obviously since the invention of pass-run options and run-pass options, reading the offensive line has become moot during most college football and NFL games.