Well some of the "other things" are things I'm not sure they can test for so I'd call those throwaway diagnoses that a lot of the alternative practioners like to use. Sometimes they can use some tests for them, but often they do something that can't really test for anything, or proves everyone has the same thing and often the prescribe about the same diet or supplements for a lot of them anyway.
Celiac can be tested for and a gluten free diet should fix that up (although a few people still have problems and may also have functional issues as that is about the same percentage of anybody you expect with IBS)
Food allergies can be tested for. Elimination diet fixes that.
SIBO can be tested for and if antibiotics make you better for a few weeks after you take them you pretty much know that is right.
Gallbladder/Pancreas can be tested for.
Most of the others what most people have that are told they have those don't get a real medical test done or are told the parasites they have cannot ever be seen in any medical test which pretty much proves you have them. (or something like that, sometimes I don't quite get the logic, but what most people mean by those are the ones doctors cannot find but you must have because you have a symptom of something).
They can actually test FOR IBS now.
http://www.ibsbloodtest.com/ there are biomarkers that distinguish IBSers from healthy controls and from people with other GI stuff when you limit the IBSers to the ones that really don't have anything else any medical test for the other things don't show.
After all most medical tests were developed to look for something else and then we think it is bad that something they were never designed to test for (IBS) doesn't show up on them.
However a lot of people will never believe there is anything like IBS and will never accept the diagnosis and will always think it must be something else.
I could give you some other diagnosis to track down if you want more geese to chase.
(and if you are female do make sure that has been checked out as some of the female problem cause GI symptoms.)
Adrenal fatigue seems to be getting popular, not the one that actually shows up as Addison's disease, but causes pretty much vague symptoms of everything.
http://www.adrenalfatigue.org/
Multiple Chemical Sensitivity is always a good one as well for any sort of just not well
http://www.multiplechemicalsensitivity.org...l%20sensitivity
Wilson's Syndrome
http://www.mall-net....rt/wilsons.html
http://symptoms.wrongdiagnosis.com/cosympt...al-symptoms.htm can give you lots of highly improbable things (as most of them do have other symptoms or clearly shows up on medical tests)