Using a specialized gauge to show wear levels on brake drums is the best way to indicate wear, however there is a visual method.
Thin taper
To give an easy indication of wear, GM builds a taper into their drums. The attached photos show this. When the drum is turned, the taper becomes smaller. Beware of drums that have been turned so much that the taper is no longer visible.
Much wider taper will allow more years of use.
