You can't rely too highly on any one factor -- performance on the field among them. A college WR's production often tells you more about his team's offense than about him. And if performance on the field were all that really counted, the Patriots would never have dreamed of drafting Matt Cassel, who had less than 200 yards passing in his whole college career...but terrific measurables. (How 'bout a 6'4" 222 lb. QB with a 34" vertical and a 4.06 shuttle?)
Film, measurables, workouts, interviews, candid chats with coaches, etc. etc. It all counts. And even so you just get it to the level of a
weighted crapshoot.