It’s not possible to self-diagnose lactose intolerance. This also includes the tests you could find on the Internet, as these tests are not scientifically validated.
Usually, self-diagnosis is based on dairy avoidance and on the presumed consequent cessation of symptoms. Yet, lactose intolerance cannot be diagnosed outside strict medical control. The proper way to diagnose lactose intolerance is to measure, in the exhaled air, the hydrogen produced by the intestinal flora after consumption of a standard dose of lactose (usually 20 to 50 g) and when one or many of the following symptoms occur: bloating, diarrhea, and flatulence. This diagnosis if performed under medical control.
When a medical diagnosis is performed, only 50% of self-diagnosis of lactose intolerance is confirmed.