You can't really tell how good they'll become but you can see how good they are skill point wise for your appropriate level!

For example I am level 4 so they players that I would be looking to improve my team would be 5* stars and from skill point wise 35 too 39 and the latter being obviously the better as you've more chance to level the player up to scout which would be 6* stars etc, etc

When you're looking for players the younger the better as they tend to be better trainers and thus better experience gainers than older players but not always the case but mostly. Some folks seem to think the price of the player also affects players ability to gain experience and hence more skill points to spend!

I've just entered the 4th level for my team which was close to an overall quality level of 35 and now my aim is to get my team as close to level 40 by the end of the season which would take my team from a 5*star team to a 6* team.

The of course there's players with special abilities but that's a whole other topic but if you see a good young player with a SA and he's at the top end of the skill point scale then that would be your best bet!

I'm sure some others could describe it much better than me and I wouldn't be surprised if there's a detailed guide or two about on this forum if you go looking but anyways I hope I maybe helped a little!