To diagnose SIBO, my doctor ordered a breath test which looks at these gases - methane and hydrogen - being produced in your gut. It's a home kit mailed to you but needs to be intrepreted by your doctor. I don't understand how SIBO can be detected via a colonoscopy or endoscopy. Google SIBO to find out what exactly it is and the symptoms one can experience (there are many) - I lost weight, felt fatigued and bloated and that my food was not being digested but just sitting in my upper stomach. Also, do some reading on testing for SIBO so you are armed with that information to share with your doctor. Let him/her know that you would very much like this test to at least rule in or rule out and how to treat if you do in fact have SIBO. What you want is to find out what is causing your symptoms, not just treat the symptoms, otherwise once you stop treating symptoms and feeling good, your symptoms will return if indeed you do have SIBO. That is what happened to me and on my third endoscopy I was told by the GI to never take the proton pump inhibitors (PPI) as they had made my digestive tract cells hypertrophic.
There is also a great book out called SUPER GUT by Dr. William Davis that helps you understand SIBO and dietary changes that need to be changed and/or added to your diet. His recommendations are very similar to what my doctor prescribed for me.
If your doctor refuses, I highly recommend you seek another one out who is willing to work with you on finding the reasons for your symptoms and treating that cause. My doctor is a functional medicine doctor as I gave up on my health providers. You can find one by city and state at ifm.org and look for one that specializes in digestive issues.
Wishing you the best in finding an answer and getting your health back.