Great article linked from Fangraphs:

An example:

The money shot:
So when do the standings matter? At the end of the season, when all the playoff spots are set. But when can we tell who is good and who is bad? It looks like somewhere in August. The terrible teams and great teams separate in June, but the middle is still muddled. This appears to clear up somewhere in August.

So how about that quote? Can a manager tell what kind of team he has within 50 games? Possibly. Can the general public tell how well a team will finish at the end of the season after 50 games by looking at their winning percentage? Forget it.