The old-fashioned way to check your odometer is by counting mile posts on the highway and comparing them to the distance the car's odometer reports.
The new-fashioned way is to use a GPS unit.
The Google Earth way lets you pre-measure a very specific route, and then go out and drive it to make the comparison.
In the google earth program, just zoom in on the area you want to measure, go to the "tools" menu, then to "measure", then select whether you want to measure a "line" or "path". ("path" is more useful unless the road you're measuring is perfectly straight!)
Use the tool to measure out your path, hop in the car, and go drive it. of course, the longer the path you measure & drive, the more accurate your odometer calibration will be.
One difficulty arises if you happen to live in an area which is not covered in a high resolution by the google earth images (meaning, you can't zoom in close enough to discern local roads & intersections).
Why bother with odometer verification? Because an inaccurate odometer throws off your mileage calculations - a 2% error on a 50 MPG vehicle = 1 mpg off. maybe you switched to different wheels or tires (or transmissions). or maybe your odometer is out of whack from the factory. Maybe you're really getting worse MPG than you think; or maybe it's better! you'll never know if you don't check it out.
If you use google earth to measure out a longer drive for better accuracy, take intermediate readings (in both G.E. and in the car) at several points along your route. Hold on to this info and you'll have a reference you can use to check changes to the car in the future (e.g. tire size, transmission changes). The shorter segments give you the option of doing quick tests.
