Alright, first we'll tackle stars.
Stars are really a loose guideline, not particularly suitable for a hard definition of a player's talent. I've had five star guys who do nothing and two star guys who give me solid seasons with Gold Glove defense. Good star ratings are generally based on the fact that the AI thinks his numerical ratings should add up to a good player when compared to his peers. He has the 'tools' necessary to perform, in it's opinion. The 2 stars for Tillman aren't coming from either your scout or the default scout opinion, but rather how the game thinks he stacks up in that league.
With Scouting on, finding a player's real talent level becomes a completely different animal. What setting do you have scouting accuracy set at? How good is your scout? How does that player compare to other players at his position throughout the league? And sometimes a scout is just going to miss on a player. In my opinion you'll be best served by looking at both what they do on the field and their ratings. Many players won't live up to their potential, others will exceed it. You just need to guess, basically.
