It’s not possible to self-diagnose lactose intolerance. This includes the tests you could find on the Internet, as these tests are not scientifically validated.
To be sure, lactose intolerance must be diagnosed. The diagnosis of lactose intolerance is solely performed under strict medical control with an ad hoc hydrogen breath test. This test includes an oral challenge with a standard dose of lactose (usually 20 to 50 g) followed by the detection, in the exhaled air, of hydrogen produced by the intestinal flora and by the occurrence of one or several/many of the following symptoms: bloating, diarrhea, and flatulence.
Often, lactose intolerance is self-diagnosed by individuals who experience intestinal discomfort after the consumption of dairy products. When they undergo the correct medical diagnosis, only 50% of individuals with self-diagnosed lactose intolerance see their condition confirmed. The severity of symptoms is generally over-reported with self- diagnosis.
Nevertheless, the diagnosis of lactose intolerance should not rule out any other underlying digestive pathology.