It's pretty difficult to answer this question thoroughly. Quickly:
QB - I don't think highlights matter much at this position. I think you need to watch full games. Does the QB panic under pressure or stay poised and make plays? Does he handle adversity well? Does he play with confidence without being reckless/careless? Does he go through his progressions and find the second and third options when the first option is covered? Does he anticipate open receivers and throw to spots or does he always wait until the guy is open? Does he step up in the clutch? Is he capable of taking over games and becoming unstoppable? A good QB always finds a way to move the ball down the field. Think about the great NFL QBs. When those guys are in the zone, you know with almost 100% certainty that they're going to lead their team down the field for a score and there's nothing you can do to stop it (Brees, Manning, Brady, Roethlisberger, and Rodgers all have this quality). That's what I look for in a QB prospect.
RB - By far the simplest skill position. Success hinges almost 100% on body type and raw athletic gifts assuming that a prospect has base levels of competence in vision and work ethic. Thin backs like Jamaal Charles and Chris Johnson must have elite speed and/or exceptional quickness. Power backs like Shonn Greene and Jonathan Stewart must have strong lower bodies and tremendous leg drive. All backs must have fluid hips, quick feet, and good balance. In general, I look for squatty players with a great first step who can change directions on a dime and run hard enough to power through bad tackles. Most successful NFL backs fit this description. Speed is not essential if the player is built strong with fluid hips and a quick initial burst. Ideal size is not essential if the player is extremely fast with elite quickness and elusiveness. Bulk is more important than height, which is actually a negative at this position.
WR - Since there are so many different types of successful NFL WRs, I think you have to keep an open mind when you're evaluating a WR prospect. In general, I've found it helpful to separate them into three categories:
Power - Big WRs. Thick and strong. Usually lack blazing speed, but compensate with size and initial quickness.
archetypes: Brandon Marshall, Anquan Boldin, Terrell Owens, Michael Crabtree, Jerricho Cotchery
Speed - Fast WRs. Explosive speed and quickness.
archetypes: DeSean Jackson, Steve Smith, Santana Moss, Joe Galloway
Finesse - Thin WRs. Generally fast, but not necessarily burners. Elite body control. Elite route running ability. Smooth.
archetypes: Isaac Bruce, Torry Holt, Chad Ochocinco, Reggie Wayne, Marvin Harrison, Greg Jennings
I'm also tempted to add a "possession" group for tall receivers who excel because they present a big target and catch everything in sight. Examples would include Sidney Rice, Randy Moss, and Marques Colston.
You might find these groupings kind of arbitrary and you might notice that some players could fit into more than one group. That's fine. The broader point is to develop an understanding of the various ways in which a defensive back can be beaten.
When I look at a WR prospect, the first thing I try to ask myself is "How is this guy going to beat an NFL DB?" Take Golden Tate as an example. Does he have the size to overpower NFL DBs? No, he's only 5'10" and not appreciably bigger than the average NFL DB. Does he have the speed to run by NFL DBs? No, he looks like a 4.45-4.50 guy who lacks a true second gear. Does he have elite body control and route running skills? He's decent in this department, but I don't see the precise body control, explosive first step, and fluid body control of Chad Ochocinco or Isaac Bruce. Tate lacks the size of a power receiver and the speed of a speed receiver. He's an undersized finesse WR with poor physical tools. He's unlikely to become a standout player in the NFL.
Now let's consider Demaryius Thomas. How is this guy going to beat an NFL DB? He's not a true speed burner either, but unlike Tate he has a physical trump card. At 6'3" 224 pounds, he's bigger and stronger than almost every DB in the NFL. He can overpower them before and after the catch, reducing his reliance on sheer speed. He has excellent body control for his size and he demonstrates the ability to win contested catches when blanketed. He will basically be impossible to cover on Sundays because he's fast enough to threaten corners downfield, quick enough to separate out of breaks, and big enough to physically dominate the opposition. He has the potential to become a standout player in the NFL.
Not every WR has to be a big. Not every WR has to be fast. But they should all have some kind of physical trait that gives them a decisive edge over the average defensive back. In general, all good WR prospects should have smooth change of direction skills, an explosive first step, reliable hands that vacuum any passes in their reach, and a well-proportioned frame that allows fluid movement. Work ethic and technique are more important traits at WR than they are at RB, but success or failure at this position still hinges mostly on raw athletic ability. Technique is largely a consequence of innate ability.
