Look for this big clue. In assessing someone for potential lactose intolerance, timing is not everything -- but it's important to note. A person with lactose intolerance will usually show gastrointestinal symptoms within 30 minutes to two hours of consuming lactose-containing foods. Diarrhea is the most common symptom, but the person may also experience nausea, abdominal cramps, bloating and gas, according to http://mayoclinic.com. Testing can tell: A gastroenterologist can perform a hydrogen breath test to determine "almost without error" whether someone will maldigest lactose, says Dennis Savaiano, PhD, a nutritional and food scientist and dean of Purdue University's School of Consumer and Family Sciences.