Well, if you're in a lab, you do a test where you say "I'm going to show you a map. Click on New York as soon as you can." etc. If you want to be more sophisticated, you add in eye tracking and see how much "scanning" people do before finding the target. I'm sure Google has both techniques available in-house.
And I'm sure if you were a little more clever you could find ways to do a similar test on normal google maps users without them knowing it.