All of us were diagnosed through exclusion. Doctors will do all sorts of tests (for IBD, parasites, gallbladder, cancer, etc.) and then when they all show nothing they say "you have IBS." Here is what I have had done (most others have had many of these tests too): blood work, stool tests, abdominal ultrasound, bone density test, upper GI (barium swallow), EGD (upper endoscopy) & colonoscopy.
You're right about
the test your doc wants you to do, SBFT - You will have to drink barium and have several x-rays so the docs can see how it passes through your digestive system. It's a way to catch any abnormality. If they are inconclusive, or doc sees any irregularies, you will be asked to have a colonoscopy.
Sorry you're having a difficult time with meds... But it's good to see you are looking further into your problems. I hope your docs can get to the bottom of it soon.
By the way, what are your symptoms?
PS - I'm sure others will be along shortly to share their stories as well.