If you're going to people-watch, Paris is one of the best cities in the world. There are so many tourists here that you can never be sure where anyone is from and who is local.
Every time I have been to Paris I have been stopped and asked for directions. My answer is usually, "Um... I think you can go that way? Maybe ask someone else". I don't think anyone is able to tell who is local and who isn't. Except for the true locals that is, I am sure they can figure you out from a mile away. When I was younger and more arrogant about my own French language ability I convinced myself that I was able to blend in as a Frenchman. I soon realised that everyone I met knew I was foreign before I could even open my mouth. "How can you tell?", I asked my friend Antony one day. "I have no idea. You can just tell. Maybe it's your hair. It's too long for a French person". My hair was so short that I could only just run my fingers through it.
I think I will just have to make myself content with having non-natives think I'm French.