First of all, don't! Like VSS said, people concentrate too much on what might go wrong, whereas actually it's the management pre- and post- that will decide if you have a successful lambing or not. top tip, if you can get yourself on a course, do so. I had to assist one of mine (a great big single out a small mummy Portland - good grass = big lambs me thinks!), and the course gave me the confidence to help out when it was needed. The other thing is, trust your instinct. If the water balloon has burst and, after 40 minutes still no sign, then get in there and help out.
I'm not a big fan of vets and would trust my friendly commerical sheep farmer for advice over the vet any day, as they would probably tell me that the ewe wasn't pregnant and that she would need complex surgery to fit a gastric band to get the weight down (arrggggghhhh money-grabbing, bull-sh***ing, advantage-taking a***holes!!!!)