There are two issues to consider here:
First, the very best HS talent should be relatively easy to identify. They have exceptional “measurables” and are standout HS performers (usually in HS hotbeds). I once got into it on a message board with one of the “evaluators” challenging the overall accuracy of the rating agency assessments. He pointed out as a example that this site accurately rated Vince Young. I congratulated him on identifying a 6’5” QB who ran a 4.3 with a riffle for an arm.
Second, there is little doubt that the top 10-20 programs consistently recruit the best talent. They wouldn’t perform at the level if they didn’t. The question is, do the recruiting sites rate the kids independent of the offers or are they heavily influenced by what programs offer a kid? This much I am sure of, Brian Kelly or the staffs of any major program do NOT rely on the recruiting sites in any way in assessing talent.