To find a hidden gem, the key is to scout a player less likely to be scouted by many others but still likely to be talented.

If a player has 10,000 ASI at age 18, he will be scouted by many. If he has 300 ASI but you have other indications he may be talented then scouting it may result in you finding a hidden gem.

Example for an indication of talent: Player owner has level 10 youth squad and is in bad financial situation (otherwise it’s less likely he would sell a real talent).

Another example: player with low grades in his main stats which you think will still develop correctly. I bought Siwiak for only 140k because his marking was 6 at the time of the purchase. But I had reasons to believe I could get it to a satisfactory level - he had a lot of tactical talent left in him according to the scout, and little physical and technical talent left. So it was clear most of his remaining training (which was considerable) would go to defensive stats.

There are a number of strategies in developing youth:

Train players you buy: high academy level, no youth squad. The higher academy level the better, especially if you have money to buy great trainees - you don’t want to waste their talent.

Train your own players: high academy and training ground. High academy essential to have talent, high training ground essential to not waste your talents - as many will bloom before 18, and by the time they are 18 and can be sold, they will be ruined if you have a low training ground level. Sophisticated buyers will find about this waste of talent and will pay much less for your players.

Train your own players and some foreign talent: need both high academy and high training ground as in above. Better choice in my opinion, because training is profitable, and training only your own talents will leave you with many empty training slots out of the 60 you actually have - a waste of potential earnings.

For a new player in the game, training players you buy may be practical. Never do high academy without high training grounds, its useless.