I'd say a near pair up seems to be a good indication. Pretty much as long as they don't go much slower (maybe even a couple points slower is ok) on their first polytrack effort, they'll usually go forward quite a bit on their second. I think that's a testiment to the added fitness they get out of polytrack, and I also think horses are MUCH happier on polytrack, so they tend to get more confidence as they run and train over it. They are more likely to extend themselves the second time out. I think when a horse feels safe and he really feels his feet on the ground he's more likely to give a good effort and do what the rider asks.