If your cow is not eating, stick a thermometer in her rectum. Leave it there for 30 seconds if it's made of glass (the thermomter). It helps to keep a string on it so you don't have to fish it out of the gutter or the cow. Read it and interpret the results.
101.0-The cow is hypocalcemic. It may be secondary to paraparturient milk production or secondary to indigestion. If she's standing, run a bottle of calcium gluconate under the skin. If she's down, you probably skipped the thermometer and went straight to the IV calcium.
101.5-The cow is normal. Either she's having you on, or she's full, or trouble is incipient.
102.0-102.5 - A mild infection, such as pneumonia. A shot of ceftiofur, often only one, will put her right in 12 hours. Ceftiofur is another miracle drug. If it doesn't put her right, further investigation is warranted.
104.0-107.0-Udder trouble. Lots of treatment options. You decide.
And that's about all you need to know. You can check the urine for ketosis, if you like. Feeding a low-level ionophore has seemingly eliminated DAs here.
Now I can just hear the handwringers and tonguecluckers: "Ohhh! These poor cows get sick?" Well, yes, they do. But the modern cow is a whole lot healthier that the old-time cows on the mom and pop 25-cow farm, whose growth and production were stunted by low quality feed, poor ventilation, and other forms of abuse. Contrary to myth, the high-producing modern cow is not "pushed" to high production. She is coaxed and babied. The HPMC is the healthy, happy cow. The cows out there scrabbling for the last blades of grass on the sacred "pasture" mandated by organic definitions are being abused. If you give that cow a choice, I can absolutely guarantee what it will be. A clean, dry, warm stall with good feed delivered, and if she can go out every now and then, she'll be happy.