Since I have coeliac disease, I know lots of babies/kids that have been diagnosed with it.

I am going to cheat and inside of typing it all out, here is a bit I found on the net.

It is important to see a doctor if you are concerned your child has some of the signs of this disease. Other, more common causes of these signs (such as infections) need to be looked for first.
Your doctor may do some blood tests and poo tests to help sort this out.
If coeliac disease is likely, the main test involves taking a biopsy of the lining of the small bowel. This means getting a very small bit of the lining of the bowel to look at under a microscope. To do this, an endoscope or thin tube is passed down to the childs stomach through the mouth. This is done with a light general anaesthetic, and is not likely to cause any problems.
Does this help? Need any further info?