You can always tell if your pig is sick by a change of its behavior. Not eating? You can take its temperature, a rectal thermometer; you should know how to take the pig's temperature. You just give it a little massage here and they will be more than happy to let you take a temperature. So I highly recommend that you get a thermometer and have that on hand. A couple of other obvious things that your pig is sick; it's got diarrhea, or it's vomiting... These things happen to pigs all the time and they can catch a flu just like us and so sometimes just withholding food for them for 24 hours will help a great deal. But do, you must contact a veterinarian any time that you feel like your pig is sick.