How do you know if the cow is having a problem calving?

It depends, really. Often if you don't see a cow making any progress an hour or two after the water sac has appeared you will need to go in to help her. If you see the hooves of the calf in the wrong position, only one hoof, only a head, only a tail, or nothing at all, then you have to go in to see if she needs assistance or not. Most cases can be corrected if you know the proper procedures of doing so, but if you find the cow has too big a calf or the calf is in a difficult position that you cannot correct, then you need to call the vet immediately if you want to save the calf and the cow.