You're kind of asking two different questions, because rookies are a special case. They have no track record so you dont have as much data to dip into (which is why only very elite rookies have big fantasy impacts, and the vast majority of even high round picks dont end up on rosters in redraft leagues).
For rookies- you cant think about starting him early in the season, he may not even be on the field very much. Once he establishes a track record you can think about starting him in a pinch, but dont get your hopes up.
For other players- think about match ups. How is the opposing pass defense? Is it likely to be a high scoring game? Does the defense have a stud cornerback, and who is he likely to be covering (this could help your guy if the WR1 isnt getting targeted)? Whats your guys trajectory looking like, is he improving week by week? Are there shakeups at other positions (particularly QB) that will affect his performance? If you are a FBG member there are lot of resources that will give you tips on these kinds of issues. A good one is the weekly passing/rushing matchups- it gives you a nice overview of how the offenses match up with the defenses and who is like to benefit.
But a lot of people get frustrated when they do all there research, feel good about it, and then things dont go like they thought. Remember that when it comes to things like this its definitely a game of percentages- you are looking for a small extra edge. If you had a better sure fire option you wouldn't be considering playing a lesser guy (oh- good lesson there, always start your studs of course but dont outthink yourself in general trying to shoehorn in longshots over established producers without a very good reason). The point is over time these kinds of decisions will pay off (if you are gauging them correctly), but any given week they may not.