Generally the arid desert / straight road / nothing for miles places tend to be Australia. Plus you can use the lines on the road to narrow it down in cases where there aren't other clues: white dashed lines in the center usually mean you're in Australia or Europe. Double yellow lines in the center mean you're in the U.S., Canada, Brazil, or maybe Mexico. Single yellow line in the center usually means you're in Mexico.
edit -- another way you can narrow down locations is just by knowing things about where Google Maps has Street View, and when the pictures were taken. If you see a picture with really bad image quality, chances are you're in the U.S., because Google was using lower-resolution cameras when they started out. The challenge there, though, is that with the lower resolution, signs are unreadable.
