Lactose Intolerance: How to tell if you have it
Diagnosing and managing lactose intolerance

Did you know that 70% of the world's population can't drink milk or eat dairy products without getting an upset stomach? Lactose intolerance, a genetic trait, happens most often in people of African, Asian and Mediterranean descent. The intolerance is caused by a deficiency of lactase, an enzyme needed to absorb and digest lactose (milk sugar). Undigested lactose lingers in the intestine and ferments, creating intestinal discomfort which includes bloating, abdominal pain, and diarrhea. Although some may be misdiagnosed as having a serious bowel disease, many people with lactose intolerance are never diagnosed at all. So don't believe you have a serious bowel disorder until you are sure milk is not at fault.
The severity of the symptoms caused by lactose depends on the severity of your lactase deficiency. People with lactose intolerance can still drink a single glass of milk without distress, although this may be different for everyone. 
Testing for Lactose Intolerance: How to tell if you are intolerant to cow's milk
If you suspect you may be lactose intolerant, try quitting all dairy products for three weeks or so. If you feel significantly better and symptoms of gastrointestinal distress have subsided, try to reintroduce the foods again to determine which dairy products you should avoid. Try some cheese or drink some milk to see what happens—you may need up to three days for lactose intolerance symptoms to show up. Your physician can also give you more definitive tests to diagnose lactose intolerance. Tests include the breath-hydrogen test and a simple blood test.
If you are lactose intolerant, try drinking smaller amounts of milk at one time. If you must drink milk, drink it with meals. Use Lactaid to help digest lactose in dairy every time you eat diary. You can also use special milk products such as Lacteeze. These lactose-free products often contain partial or no lactase enzymes—be sure to read labels carefully. Lactose-free formulas are available for infants with lactose intolerance.

Getting enough calcium is a problem for those with lactose intolerance—drink calcium-fortified soy milk to get the right amount of calcium daily. Try calcium-fortified orange juice as well. Dairy products are sometimes low in lactose—examples include cottage cheese and Swiss cheese.

Always check labels for lactose, as many commercial products contain lactose. If the ingredient list contains the following ingredients, it has lactose: whey, curds, milk by-products, dry milk solids, and nonfat dry milk powder
Lactose Intolerance: what about yogurt?
Studies showed that the active live bacterial culture (also known as probiotics) found in yogurt aid in lactose digestion. Frozen yogurt, however, does not seem to provide the same effect for many people. As everyone’s lactase deficiency is different, you might not know how you will react to a product until you try it.