The most obvious one here is accent. The only people I've ever met that I assumed were from here but actually weren't turned out to be from Cape Breton, Waterford/Wexford in Ireland, and the Inverness region in Scotland. And even then, most people from those three places do stand out a little - I've only been truly fooled a handful of times.
Without speaking to people... most of the obvious tells here are a little embarrassing. If the person is exceptionally tall or not white, the odds - though changing - still suggest that they're not from here.
Anyone who uses an umbrella ever kind of stands out as a tourist. Locals have near-universally given up on them due to the wind. You really only see locals use them at fancy functions - Memorial Day, state funerals, whatever else. And if I see someone using them for snow, I'll go out of my way to find out what part of Eastern Europe they're from just to keep that association strong in my mind.
My fave one is actually a gay versus straight thing, which of course isn't accurate but still fun. If you ask a guy to show you his fingernails, and he spreads out his hands like he's laying them palm-down on a table, slip him my number.
If he has his palms up, fingers curled back almost like a fist, well I mean you can still slip him my number but I won't hold my breath.