How do i know if i'm lactose intolerant?

If slight symptoms..: A person can drink lactose-free milk for 2 weeks (and also avoid lactose foods), and then "normal" milk for 2 weeks, and then lactose-free milk again for 2 weeks, and finally go back to "normal" milk. If a person has gas, bloating, with or without tummy aches when on "normal" milk, but not when on a lactose-free diet, then there is very likely lactose intolerance. There also is a lab test.