When I travelled to Buenos Aires for vacation a few years ago, I was mistaken for a local within 3 days by someone who was asking directions. In Edinburgh, it's taken 8 months to get there. Funny considering I look much more Scottish than Argentinian. As mentioned before, no one thinks I'm American, but no one ever thinks I'm Scottish either. It's the accent, of course.
The story goes that a guy approached me at a pub, asked if I was a local, and I nodded yes to see where he was going with it. He said "It's rare to meet other locals, isn't it? This city is so overrun with international students." Before his foot went in his mouth any further, I let him in on the truth.