Complete dumb luck! I'm such a nerd I actually started reading about the chassis they are bringing to the tracks though and that really helps. When I see well this car finished 18th and 16th this year I likely won't be picking that guy. Also, I have started paying attention more to 10 lap averages instead of overall fastest practice times.
But, at the end of the day to get the top 3 in order is just really stupid luck.