How can the doctor be sure i have ibs and nothing more serious?

Should test more : Irritable bowel syndrome is usually a diagnosis of exclusion. You first need to be tested for more serious conditions (i.e.: inflammatory bowel disease) before declaring its irritable bowel syndrome. Seek a second opinion from GI especially if having blood in the stool, weight loss, abdominal pain, etc.. Best of luck!

Never sure: IBS is a diagnosis of exclusion and there are specific tests designed to rule out common treatable conditions. In order to formulate a putative diagnosis of IBS these tests should return negative. Providing this is the case functional gastrointestinal conditions remain poorly understood and described and a variety of overlap conditions are seen in specific individuals. Hope it helps.