There are spreadsheets on the forum to help you out with it.
Here's a general value interval for 18 yo 99 % q players to aim for: https://forum.topeleven.com/tutorial...tml#post500463
Here's a more detailed spreadsheet you can download and identify potential fast trainers based on level, age and quality (%): https://forum.topeleven.com/tutorial...tml#post662448
Here's a training calculator to set up your training properly and efficiently: https://forum.topeleven.com/tutorial...-interest.html
Here's a guide as to how it works (powertrain your fast trainers in order to maximise the rest pack efficiency): https://forum.topeleven.com/top-elev...tml#post529698
The key to identifying a FT is to first compare the player's market value to his overall quality when you buy him, like in the first and second links. That'll present you with the possible/potential FTs. Then, you have to test the players with drills/sessions either by adding a SA/NP or giving the player a training session of 15-18 % condition loss like described in the first link or target only one or two white skills like it's described in the last link. If you want to be sure, try all three versions. That'll confirm whether the player is a FT or not.