Detecting Browser Back Button after using history.pushState

I have a three-stage login form that shows/hides content on the page as progress is made. When the user proceeds from step 1 to step 2, I call the following:

    var stateObj = { foo: "bar" };
    history.pushState(stateObj, "", "");

And I see the browser back button enable. Now, I'm trying to catch the back button click so I can hide/show content (for example - back to stage 1) accordingly. How can I detect the browser back button in this scenario? I don't want the url to change, I just want to call some JS function when the user hits back. I am targeting modern desktop/mobile browsers.