Yeah I know, you must have 2 players and train them until they reach 129.3 M. Whoever gets there first (with a lower overall) will be the faster trainer. But thats hard to pull off. The best tactic for me is to train a special ability and see how many times he gets + 2 in a set with 15% condition loss. (I think you mentioned this somewhere here in this thread as well)
So basically you need to buy players and do testing with them to determine if they are fast trainers. Because I'm only buying 3-4 star players for like 3 tokens max it is not a big problem. You also get a general idea when you sort by value in auctions if someone could be a faster trainer. Also youth players are always appreciated, you pay a few more tokens but they are 100% faster trainers.