I too have diarrhea everyday for 4 months straight now and I have had MANY tests to confirm IBS with diarrhea.
Most likely, you will have a colonoscopy where they look at your colon (large intestine) and an EGD, where they look at your stomach. You are not awake through these tests. I also had a small bowel series x-ray, upper GI, hida scan (to check the gall bladder), CT scan of abdomen and pelvis, ultrasound of pelvis and gall bladder, number of stool cultures, blood tests, etc. This is how a person gets diagnosed with IBS. If the tests show nothing signifigant, then most likely, you will be Dx with IBS.
The colonoscopy and EGD wasn't bad. I don't remember a thing about it. The worst part was drinking the prep the night before and the bathroom time that went with it. I hope this helps you!