使用浏览器的“BACK”功能触发脚本（js或jQuery）

I can't find a way to execute a script when a page is reached via the browser's BACK button or key command. (Background: I need to know if a page was opened using the browser's BACK button or key command. Then I could check for a stored sessionStorage varible and trigger some appropriate stuff). I can't find a way to execute a script when a p