You inspect the core by feel. Does it have weird bulges? Hollows? Is the cover doing something weird? Unless you've put chemicals on your line, polyester and nylon wear in predictable ways, and in the case of kernmantle line, hardly wear at all with an intact cover. As always, YMMV. I do what I do, and others can choose to follow or not, but it'll definitely be a cold day in hell before I pay $30 for a piece of rope blessed by a manufacturer, when I can buy the same rope for $6(being very generous on the high end here).