Its a very simple trick
what you need to do is before you turn your back to the table just remember what the ball is on the left , lets say its the 3 ball so this is how the balls would look before they moved any
3 4 5 6 7 8 9 10 11 12 13 14 15 1 2 keep in mind the ball on the left is the 3 now you turn your back and let them move any amount they want, lets say they move 6 balls now they will line up like this
9 10 11 12 13 14 15 1 2 3 4 5 6 7 8
now with your back to the balls feel and count to yourself 3 balls from the right and the answer is "6"
As long as they dont move the ball out of sequence it will work everytime.... Just dont forget what the starting ball is on the left
PS it works better if you dont make it look too easy, take your time to give them the correct answer, even start to move the wrong ball so they think you dont know the answer then grab the correct ball and roll it foward. I use to do this trick all the time, kept them stumped for days and they never figured it out till I told them how to do it