I am not sure I understand.
One doesn't need to have played or coached at the college level to assess performance.
If that were the case, every University president, athletic director, professional owner, or general manager would need to have played at the relevant level. While some have, most have not.
In fact, the best teaching coach I ever saw was a Norview High head JV football coach, and head swim coach. He never played HS football but regularly produced undefeated teams. He also produced stellar swim teams and couldn't swim well enough to even safely go in water over his head.
Having said that, it would seem to me that many fans can at least be able to discern athleticism and skill.
For example, if a db keeps footraces with opposing players, it might be that he is not as fast enough to play that position competitively. I think most fans could see that accurately.
If a WR consistently drops passes, I think fans can see that.
A QB does not throw accurately, I think fans can see that.
If an interior lineman consistently gets beaten, I think fans can see that.
If a FG kicker cannot make a FG, I think fans can see that.
It may not always be apparent the root causes of the issues.
That is where the luxury of forum speculation comes in. Without it there is no reason for these boards at all.