You know when you are obessed to Doctor Who when you want to paint the door to your room like the outside of the police box so your room can be a TARDIS. And the color of your door has to be TARDIS blue.
When you pass a statue and you can't help but to force yourself to not blink in the silent fear that it might be a weeping angel..
having two shadows is a death sentence OR (my personal favorite) shouting "ALLONSY" or "GERONIMO" when the plane takes off
When the moment you get your toy sonic screwdriver in the mail, the first thing you do is sonic your phone to give it universal roam :D