Here is some information from a guide who writes for eBay. The citation to one of his web pages is below.
The eBay Guides are invaluable - whenever you are trying to find out about some object, those guides tend to be quite reliable, written plainly by experts.
"First of all lets identify what is classified as a "Low Vein" cap. A genuine low vein hat is going to be manufactured by the Mine Safety Appliance Co. (MSA) A true low vein is classified by MSA as a type "G". These hats (in low crown design) are no longer produced, hence the value they bring. Modern plastic Comfo caps are not true low veins. They are advertised as a low crown hat but when compared with a vintage comfo cap there is a considerable amount of difference in height. If the hat in question does not have the unique light brown/dark brown striped colorization it is not a true low vein and is only worth 10 or 15 bucks. "
"Most true low veins will have a lace in liner with a very few having the snap in liner. If the hat in question has a lace in liner then you are on the right track. True low veins will not have anything imprinted on the underside of the bill of the hat. Some of the older low vein hats will have built in flip-down eye protection attached to the bootom of the bill. This should not affect selling price because they are removable. A true low vein will measure 4 and 1/2 inches high. Look to pay more for a hat that has no scratches or cracks or repairs. The presence of stickers or paint should have no effect on the value. They can be removed if wanted. Be ready to pay up to $250 for an excellent mint condition low vein. Buying a cracked up hat for $100 is just a waste of money. The hat can no longer provide sufficient impact resistance. As a side note there are some people who will take a newer Comfo Cap and actually remove material (reffered to as "cut down" caps) from the bottom of the hat to create a lower crown. Not only is this illegal (MSHA rules and regulations for approved head wear) but also dangerous because this creates a brittle and unstable hat. I hope that this guide can help you in purchasing or selling a low vein hat. "