Anyway to test MPG on Honda Civic Hybrid without asking dealer to fill tank?

Is there any real way to tell what MPG I will be getting before I buy the vehicle? I have read that some 05 civic hybrids get 30 mpg, and some claim they get 40-45, without having to ask the dealer to fill the gas completely, drive it around for 40 miles, and do back to the gas station and see how much gas I have to put back in, is there any way I can reset the Trip gauge so that it will automatically calculate it for me as I test drive the vehicle for say 30 or 40 miles? (I think the vehicle has this feature, I am just not sure how it works)? Thanks again, this will be a huge help, Currently getting 13 mpg in my truck and want to go to school, but it wont happen unless I get a good reliable gas conserving vehicle :).