I had expected someone else to jump in here.
The more or less usual signs are the tail head becomes very prominent as the muscles on either side of it relax, the vulva becomes very loose and floppy, she may or may not bag up and frequently she will get a look almost as if she's calved because the bulg in her right side appears to go away. The bug going away is the calf rotating into position for delivery. There will be a discharge of a runny looking fluid from her vagina. Shortly before calving she will usually remove herself from the rest of th herd. She may appear very restless, pacing, laying down and getting up. Her tail may or maynot be carried in such a way that it looks like a pump handle. If she appears to be working at it for over an hour and nothing happens, check inside and make sure everything is ok. From the time the water bag appears until the calf is on the ground should be no more then an hour or so. Once the bag appears, if no there doesn;t appear to be progress at a fairly regular interval, every 15 minutes or so, glove up and check to see if something is wrong. The surest sign that calving is eminent is when the feet of the calf appear.
There are 2 times during late gestation when you may see fluid. The first is when the cervical plug is passed. This fluid is sticky and tends to hang on and stick to whatever it touches. The second is more watery, not sticky and is the lubricant fordelivery.
If you can get the boolean search to work on this site or even google you can find tons of information on when and how to assist
dun