The hairs of pig are very hard and when they are removed from the skin they leave a distinct hole mark on the leather. Secondly, these holes are in group of threes. Which means that you will find group of three holes and these groups are at little distance from each other. On the other hand the cow or lamb skin the hole marks are not visible and if at all they are visible they will not be in group of three holes. They cannot be seen visually.