The gold standard test for lactose intolerance would be a Hydrogen Breath test. The individual would be required to fast for a number of hours prior to the test being done. They would then breathe into a specific apparatus which would measure their baseline hydrogen output in their expelled breath. They would then be given a high lactose liquid to drink and measurement for hydrogen would be done at intervals and ascertained as to whether or not they are digesting lactose efficiently or not.