I don't have the same problems some people are mentioning about potential. Your own judgement on attributes plus scouts advice should give you a pretty good idea of a player's future. Don't forget, I think when the scout says "currently playing close to his full potential" it's really misleading. They should not have used that wording, normally the player can still improve quite a bit but is probably already fairly good for his level. If they can't improve they will just say the more obvious "won't improve" message or the "some way off" message if they won't improve enough to be worth it.
Also, like was already said, the number of stars given is related to your team's league/stature/current player quality/etc (I don't know exactly what it is, but you get the idea). When I want to know the potential, I just compare the difference between current and potential star ratings and use his current stats to make my own judgement. If he's got a good base of the stats needed for his position and can improve, even just by half a star, he's worth at least a closer look. Half a star is a pretty good improvement actually.
I've read (in the in-game tips) that scouts give more accurate reports the more times they scout a player. If you're willing to wait a few weeks in-game, try the scout for 3 matches option if you're undecided about a young prospect; I'm sure it will be a more accurate indication of potential. I've never bothered though