Each individual heat pit has a thin layer of skin, and under this skin are many nerve endings that are very sensitive to heat. But scent plays a big part in it as well or else pythons would perhaps strike at anything that gives off heat.As you've probably noticed, they also have pits under and around their nostrils. I believe that these work in combination with the pits along the bottom lip to help not only detect prey, but to determine the distance between the prey and itself etc. Just my opinion, though.