What if from one session he got 7, and from the next one nothing? It seems you need the average gain from several sessions.
This is not an answer, and I'm sorry in advance for that, but you need to spot FTs before buying, not after several training sessions. Your question is good (I don't know exact answer though, I'll do some tests after my Academy gives birth). But it's not the question you should ask, if you don't know how to tell if the player is FT or not.