How can I determine whether I'm lactose intolerant?

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.