Lactose intolerance. If regular milk consumption causes stomach bloating, cramping or loose stools and lactaid treated milk does not, then you are lactose intolerant.
Trial and Error. If your symptoms disappear after a week of a totally lactose free diet and return when lactose is started again, then it is likely. Scientifically, an intestinal biopsy or a breath hydrogen test are generally accurate measures, but much more difficult and invasive.