Over at the incredibly-useful InDesignSecrets Blog, my friend Anne-Marie has posted a concise visual reference to all of InDesign's on-screen hidden characters and markers. These are InDesign's visual clues that indicate that you've entered a space, non-breaking space, line break, tab, or other difficult-to-recognize character. In order to see these characters on the screen, Type > Show Hidden Characters must be chosen.
In this well-organized, downloadable PDF reference, Anne-Marie shows 43 different doo-dads that may leave you scratching your head when you see them on the screen. Now you can just look them up in this handy reference. Thanks Anne-Marie!