When "sorting" lentils, what exactly am I looking for?
It is not unheard of for small pebbles, or pieces of the pod the lentils came in to find their way into the drying process.

There is also occasionally a lentil (or bean) that was a bit dodgy before it was dried. These usually appear as discolored lentils, which should be thrown away. It is rare for more than three or four to appear in a pound of lentils though, and the drying means that one bad one doesn't affect the rest.

The same is true for beans and chickpeas too, but is rarer the larger the beans get.