Good shepherds spend time with their flock, just being with them and quietly observing. Sheep being prey animals, they're very good at hiding symptoms. So you have to know them well enough to know what's normal, so you can pick up on not normal.
One of my examples is Mildred. To someone who didn't know her, she'd have looked like all the other sheep that night. To me, Mildred not mobbing the quad bike and running alongside with her head inside the sack being first at the cake was not normal. So I went back to check her later, and sure enough, she was down. Calcium and vitamins, and she was up and running within 20 minutes. If I hadn't known that behaving like all the other sheep wasn't normal for Mildred, she'd have been dead by the following morning.