TRansfers come more or less down to gut feeling.
In FM2015 the scouts report current ability CA and potential ability PA.
The PA however is often highlighted with golden stars that show known potential, and the dark star that shows potential potential.
to reveal how much potential they realy got you got to extensively scout them.
Personaly i look at abileties for the roles and take scout raport into consideration.
if the player has a mentality that fits the team i will go for it, adn altho this ocasionally backfires as when i bought vadillo that had potential 5star potential and turned out he was only 3, im quite pleased with the signing for my save as he was spanish and had good abileties for the role as AMR.
Just trust you're gut, play teh game and as you develop knowledge of players you will know more about what is a good signing and whatnot.
I would advise that when signing youth you avoid players with less determination than 12, and i personaly wount sign any unless they got 15.
hope this helps