How to spot a time traveller

I have a few talents, and although it clearly betters the world by pointing out things that look like other things... I'm focusing on how to spot a time time traveller.  Here's the few I've run into as examples to help you do this at home.

Someone needs to call Marty McFly right away.. I'm a little worried time travel is super cheap in the future if some guy travelled back in time for the coffee.   What do you mean, he's not a real person?

Why did I base all my sci-fi knowledge on 80s movies.