If you sort them by age, you can see which "older" guys are better trainers, if you compare their values and see that for the most part the higher value means faster train... but not always, so you can see which are slower than expected too. I do that to see who maybe isn't going to gain enough Q to keep up, and those guys are sold if they get injured, unless they are awesome players.

When you see who consistently gains a lot, those are the guys you can consider add Role or Ability to, and worth giving extra training.

There are a few that got over 20, those I would consider fast trainers for sure. But what they gain does vary, you can still tell who is doing well by comparing it to the average listed on the training.. anybody who does 2x that is pretty good gain.