The symptoms of irritable bowel syndrome vary from person to person. Some people get constipation, some people get diarrhoea, and some people get both of them but at different times. IBS is diagnosed based on the symptoms - there is no test to confirm IBS but tests can be useful to rule out other conditions.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/what-are-the-symptoms-of-irritable-bowel-syndrome