Almost Uncirculated 58 (AU58) is a confusing grade and in some situations, depending the coin grading service or the grading expert, one day a an AU58 might be considered MS60 or an MS60 could be considered an AU58. My statement might seem unfounded to some but it's true, there's dealers and collectors that buy every graded AU58 coin they find just so they can resubmit, in hope they get an upgrade to MS60.
An AU58 coin is graded such because only the very highest points of the design have the slightest amount of wear. If you know Morgan Dollars then you know that they can often be found with weak strikes on the high points, furthermore they were stored in stacks, one on top of the other. Silver is a metal and all metals can be smashed, however slightly, under their own weight.
It's these above two facts that must be taken into consideration when grading a barely circulated example. The true secret of properly grading an AU58 verses an MS60 example is luster. On the obverse look closely at the hair of Liberty's ear and over her forehead, and on the reverse, look at the breast feathers area. Don't worry if there's missing details because it could be a weakness of strike.
When examining these areas you should be looking for a difference in luster, and a circulated AU58 Morgan will have a slight dullness in luster in either all or at least one of these areas. It's best to get several opinions or a top tier grading service's opinion, especially if your coin jumps in value from AU58 to MS60, and to be weary of buying over-graded coins just for that reason alone.