How can you tell if a 10 yr old child has muscular dystrophy?

See a doctor. If you are at all worried you should see your physician. Typically kids will have difficutly getting up from a seated position, going up stairs, and keeping up with their peers. Some are toe walkers. Many things can mimic muscular dystrophy, so it is important to see a knowledgeable professional who can help you sort it out.
Blood work. Besides clinical muscle problems, there are accurate blood tests that confirm the diagnosis.