Mileage

I just recently purchased a 2006 Chevy Trailblazer LT. Its a V6 Automatic with 20K miles on it. I bought the truck for a very low price but I am beginning to believe it has mileage discrepency. The body is in really rough shape and the rest of the truck looks like it has been driven many more miles than it shows. I am starting to have some mechanic problems as well. Is there any way for me to tell if there was odometer fraud? Other than the vehicles service history (its clean). Does mileage get stored anywhere else, other than the gauge cluster, that I (or a mechanic) can access? Thanks so much in advance for you help.