IBS is a functional disease. A functional disease is an illness that occurs despite there being no apparant physical abnormality. An IBS diagnosis is usually a process of excluding more serious ailments. Here are some of the testst that need to be done to arrive at a proper IBS diagnosis.

• Basic blood tests for common problems like liver issues, nutrient deficiencies, anemia, etc... Make sure your iron level is checked to rule out Haemochromatosis(too much iron in the blood).
• Stool test, to check for blood in the stool. In many cases this can be done at home with a take home kit.
• Lactose test, to see if you're lactose intolerant or you can give up dairy for a few weeks.
• Colonoscopy & possibly an EGD(Esophagogastroduodenoscopy - yes it's a crazy long word!), one is for your rear end, the other is to check your stomach. EGD may not be needed if you're only experiencing lower digestive problems.
• Abdominal Ultrasound to check for gallbladder problems.

You should be adamant about all these tests. Do not accept them telling you one is not needed. Endoscopy tests cannot be done by a General Practitioner or a Family Doctor - or at least they shouldn't be! They are usually done at a gastroenterology center or a hospital by a trained Gastroenterologist. Ultrasound tests may vary, I had mine done at a hospital with an ultrasound technician.