Talent Evaluation Factors for Offensive Linemen (OL)?

Compared to individual baseball and basketball position performance, it is harder to quantify performance of football offensive linemen. Review of game videos or film is used to evaluate performances of individual player's assignments. Some screening of college players prior to the NFL draft such as all conference by position, NFL combines, scouting reports, etc. give some notion as to how well players will perform as professionals. The occurance of undrafted free agents as pro-bowl selections as well as lower draft round players success is a point in fact that the process of screening and drafting players is flawed. Another comment is that early draft picks are overpaid given that they haven't yet proven themselves at the NFL level. On the other hand, the talent of experienced NFL players is evaluated in terms of the value of their contracts. Yet even for experienced players such as free agents they can also be overpaid. For instance the value of Albert Haynesworth's contract is subject to debate and his influence on the effectiveness of other players on the team is part of the debate. Similar arguments are made for the impact of an offensive line's performance on their quarterbacks effectiveness.