You are just looking for the tools. There is no way that in any college offense, spread included, pro scouts know what is happening in the huddle, the play that was called vs what actually happened, and so on. You are looking to see tools:
How is Percy's route running? What kind of routes is he running/can he run effectively? If he's only running a limited number of routes (which is the problem with a lot of big-time college receivers), how does he run them in your personal workouts? The key to getting open in the NFL and running good routes is making every route look the same to the DB. Does he have telling signs in his breaks or cuts? How does he get off the line of scrimmage when there's press coverage? Things of that nature.
Is he a coachable kid? What is his attitude. This is where the personal interviews and talking with his college coaches and family comes into play. Is he a team guy? If he runs poor routes, is it because he wasn't asked to do so? Can he learn/is he willing to listen?
How are his hands? This is hard to determine based on the different type of QB the WR has throwing to him. Where is he catching the balls? Are they right in his hands every time, or does he have to constantly reach out/up/down/behind? What is the arm of the QB like? Is he catching balls from a weak armed QB or a strong one? There are so many things that matter regardless of the style of offense he is in.
These are off the top of my head, I don't pretend to be a pro scout, but regardless of what system they run, you are looking for these types of things. Obviously it help to tailor fit players in similar schemes to your NFL style offense, but its not the end-all.