How to tell if you are lactose intolerant

The Lactose Intolerance Test:How to tell if you are intolerant to cow's milk
If you suspect you may be lactose intolerant, stop drinking any milk or eating any dairy products for at least two weeks.
If you feel better - and the gastrointestinal symptoms have diminished - you can do a "challenge" or "reintroduction" test to try to determine how much of which dairy foods you need to avoid. Drink a little milk or eat a little cheese and wait for two or three days to see what happens. It may take that long for symptoms of lactose intolerance to show up, say experts.
Your physician can also give you more definitive tests to diagnose lactose intolerance, including blood tests and a simple breath-hydrogen test.