Lactose intolerance diagnosis
The most common way of lactose intolerance diagnosis is the hydrogen breath test. In this test the candidate is given a glass of pure dissolved lactose to drink on an empty stomach. Afterwards they are asked to breath into a hydrogen detecting device in given time intervals (e.g. every 30 minutes), until the diagnosis is confirmed or, if long enough time passes and no result is observed, denied.
Other than this a blood glucose test is an option but it should be compulsory to the breath test.