when i bought my last bike taking it out on the trail wasnt an option as you say...its hard to tell much about a mtb in the parking lot but what i did is just ride it up some hills over curbs and the most important thing that you can tell is just the feel of riding the bike as far as how you set on it and just if the bike feels like it was made for you...you may have to ride several to tell which has the best feel but this is probably the best you can do when a demo rife isnt an option