What are some tactics I can use for finding the gems amongst the huge pile of regens? What kind of stats would they have at a young age and how do I know if they have the potential to become great?
The most important thing is to get a network of scouts capable of being able to spot both potential and actual ability. Scouts have a stat for each of these on their details page. Obviously, it depends on your budget but
20is the max stat for this so 15+ is usually a good bet.
Next, its mainly luck of the draw and sending them scouting in the appropriate areas, lower leagues, U18 leagues, abroad, U21/U18 continental competitions.
Your scout will be able to report on the best players in that competition and usually send you updates. Obviously, the longer you scout a particular player the better understanding the scout will have of the players potential.
Players can attain up to 5 stars in terms of potential ability. You want to be looking at players with 5*/4.5* potential but its important to also be aware of their current ability. Player's in game development falls dramatically past a certain age so if they are 20 and their current ability is low, it is very unlikely they will ever reach their potential.
You will then have to nurture this talent appropriately, giving him a role model within the club that plays in a similar position or style, play them in games to develop or send them out on loan.
All of these factors will weigh up and given the right amount of time you will be able to find top class players (not necessarily like Messi) but still.
NB: There is also a program you can download that runs in parallel for FM2013. It is called FM Genie Scout which basically queries the database and returns a load of readable data for you. You can select a position and then order by the players with the most potential in this position and buy them (even tells you how much is likely to succeed in an offer). They are aiming for a 9th December release for this