How will my doctor check for lactose intolerance?

Your doctor will use one of these tests:

Blood and Breath Tests

You will drink a sweet drink with lactose in it. Then, your doctor will test your breath or blood for signs that you did or did not digest the lactose.

Stool Test

Your doctor can also find out if you digest lactose by testing your stool (bowel movement). The stool test is often used to check babies for lactose intolerance.

source: http://digestive.niddk.nih.gov/ddiseases/pubs/lactoseintolerance_ez/index.htm