impact of position coaches in developing players

When Ron Cooper was hired, I remember reading comments here, like "Oh, he's a good hire. He helped 6 kids go pro from USC, and that program doesn't have a lot of talent." That made me start wondering just how much credit coaches, and specifically position coaches, deserve when their players go pro. Actually, every time I see a coach's resume and it says how he coached all conference or all American players here and there at schools along the way, it makes me wonder this.

It is fairly easy to see the impact of head coaches and how their decisions and strategies move a program for better or worse. The impact of position coaches is a little less visible, and some are hired for technical prowess, others as recruiters. What if a position coach is a great recruiter and a lot of his guys go pro because they are talented, even though he doesn't teach technique very well? How much credit should he get for having developed them? How do you know if it's a function of a borderline or lower talent being coached up, or if it's a function of luck and signing under-rated talent that makes you look good?