Well, the most obvious of the advices would be having a roaming scout looking for young talent in each of the big footballing nations. A sneaker way is checking the player ratings for the youth players in some of these teams. Chance is, the best players will have the highest average ratings. Problem with this method is that by the time they play 6-7 games so that you can get a good idea of their performance someone like Porto, Milan or Barca might poach them. Which brings me to the third, sneakiest option: check the transfer window of the worst poaching offenders every week or so. I stole a few talents right from the desperate claws of said clubs with Manchester United a few times, when they were already negotiating with the player.