I've asked my doc the same thing. If you've had a colonoscopy that shows UC inflammation, then that's what you have. Now that's not to say that you don't ALSO have IBS. If that's the case, letting the IBS go untreated can make your UC worse.
I don't know of any conclusive test for IBS, but if you go on an IBS diet and see improvement, that should show you something. The website I have posted in my signature is a great site to show you how to eat with IBS. Maybe you could try out the tips and see if it makes a difference?
DX: Pancolitis as of 9/08 (Proctitis as of 1/08) and IBS
What Worked: Spinach/Sunflower Seed Diet & IBS Diet (www.helpforibs.com)
What Didn't Work: Canasa suppositories, Cortifoam, Prednisone, Colazal, Symex DuoTabs, Rowasa, Bentyl, Cortenema, Asacol, Iron, Zoloft, Acidophilus.