slow or fast trainer

i dont understand how can you tell which player is a slow trainer and which a fast trainer?

thanks

By their price..

Say an 19 year old 34 quality player is worth $4.59M and a 18 year 34 quality player is worth $2.56M, then it means the more expensive player is the faster trainer. You can compare their prices if they are close in quality and then you can see which one is faster