That highlights lesson one of my continuing life course of study in stockmanship. It is, simply, go out and go out often to look at, listen to and really study the animals in your charge.
I was raised in a house full of books and given a pretty broad view of my world from the seat of an old Studebaker pickup, atop many a sale barn gate, and perched on straw bales at livestock shows and breeder auctions. Dad began and ended each day with the stock, and I believe he could eventually spot one just when it was starting to get sick.
Thirty minutes just before full light and just before sunset are optimal times to walk among the creatures in your care. During those times they are generally more closely grouped, are settling in or rising up from a night of rest and are more easily approached for closer examination. These are also times when livestock are more vulnerable to predation.
I find much benefit in watching hogs rise up and come off of their beds. It is at that moment that they will demonstrate the earliest signs of lameness, their feet and legs are most observable, and they will often then demonstrate early respiratory ill precursors in the form of coughing, sniffling and/or labored breathing. Those last animals off of the beds and slow movers should be noted for further observation.