Keeping sheep healthy is not very hard. So long as it has enough suitable feed, (vacinations and worming if you're not organic farming) mineral and salt, has a chance to get out of the wind and rain (and even that isn't a requirement in warmer climates) sheep stay healthy. They will still get sick despite your best efforts from time to time and eventually you'll need outside help. Your vet may coin a cute saying something like this. There's a 4 "S" rule to sheep farming. Sick Sheep Seldom Survive, you may not see the humour in that, I know I didn't. The point is vets see too many hopeless cases far too late to for them to help. Sick sheep need prompt attention!
So you have sheep that looks poorly, at least to you it does. You can't find or think of an answer so you're going to call the vet. Great, use it as a learning experience too. Most vet calls here are if I'm worried about flock health because of the sick sheep or am really stumped. It can be hard to get a vet to examine a sheep, so if you call armed with relevant information that shows first, you're on the ball and second, that he/she isn't wasting their time making that vist to your farm, you just might have better luck. Large animal vets rarely stay in the job for the money, they want to make a difference and enjoy the work. Hopefully it won't be a big problem getting a vet out although I've heard of some horror stories. (There is a phone number to Pipestone Vet Clinic that will answer sheep health Q's free of charge in the Sheep Basics post if you're really stuck) What does the vet need to hear from you up front? The breed, sex, age, status ( bred, open, with "X" month old lambs) vacination and worming history and any other meds, temperature, eating, rumination, and manure charactor, feeds and changes to feed, body score, and noted symptoms of the sheep you're calling about. Sounds like a lot but only takes a few seconds to rattle off most of the time certainly less than it took to take it's temperature. Hopefully the vet is suitably impressed (or at least curious about you) and is coming. How do you greet him/her? A nice bucket or two of warm water and a couple of towels ready wouldn't hurt, the sick beast penned in a well lit easily accessable area even better!

You should have a notebook and pen ready and be dressed to help. Offer to help! Manure samples can be helpful, or anything else relevant like bloody milk etc. If the sheep has a medical history it might help to have it handy too. I try to keep a phone close too. Have a knife and rope handy, if you have shears to clip the wool have them handy too. Ask questions, you're hiring the brain too so work in lots of questions however they're related to the sick animal. If you're hearing too much latin say so and ask for clarification! Use that pen and notebook to write some of the advice down too! It's easy to forget in all the fuss but try to keep some notes going! If there is medication needed write down the instructions and ask about withdrawl times even if you're not going to ship the animal. (It's kind of an etiquete thing, just the right thing to do!) Ask about quarenteen times from the rest of the flock, and remember the sheep was part of the flock when it got sick so ask about early warning signs for the rest of the flock and preventative measures. A short reveiw of your managment in effect. That might even warrent a seperate call to discuss an improved plan!! Ask what you should expect for the sick animal and for any supportive help you can give. It's a good time to get a demonstration for tube feeding eleltrolites etc. if you've never done it before!
Hopefully the sick sheep is on the road to recovery but make follow up calls if things change, even if the sheep has recovered.
That's a lot to read but hopefully it helps.