Not accurate enough to know for certain if the dog will pass certification as an adult, be breed worthy, etc... But any dysplasia severe enough to have a significant impact on the dog's future quality of life or ability to work is likely to be evidant at 6 months old.
Knowing if there is a problem before you buy the dog makes your decision easier. I personally would not buy a dog if the prelimes showed a problem. Also things can change as they get older even with the prelims showing no problem.