As soon as I see an acoustic guitar, I know I've accidentally stumbled upon folk music...and I run the other away, look for the chips 'n' dip!

Unless of course the person starts beating on the guitar, and then I know it's flamenco. I once started unconsciously beating on a borrowed guitar when I was singing/playing Jamaican rocksteady...suddenly becoming aware of the horror on the owner's face...it was an antique "Martin" or some such thing, heh.