Screen.fullScreen is a bool property that can be read or set. You can use this to determine if the player has just left fullscreen and switch to a different resolution.

example:

private var wasFullScreen : Boolean = false;

function Update () {
    if (wasFullScreen && !Screen.fullScreen) {
        // this gets executed once right after leaving full screen
    }
    if (!wasFullScreen && Screen.fullScreen) {
        // this gets executed once right after entering full screen
    }
    wasFullScreen = Screen.fullScreen;
}

edit:

If you're building for the web player you can disable the right click menu so people can't go fullscreen to begin with.

To get to the setting go to File > Build Settings, then choose Player Settings and select No Context Menu for the Web Player Template.