Most people who are diagnosed with IBS and eventually realize they have UC or Crohn's are certain that they had had the UC or Crohn's all along, but were simply misdiagnosed with IBS previously. Some people have reported that their IBS symptoms went away completely after a certain amount of time or a major change in their lifestyle or treatment program. Other cases have reported having to deal with IBS for life. Again, it depends on the individual. The ONLY way to be absolutely certain you have IBS is through a doctor's diagnosis.
Tests given are almost always for the express purpose of ruling out other afflictions.
There is no test for IBS itself. If you suspect that you have not had a thorough enough examination for other afflictions before the doctor tells you that you have IBS, you should seek a second opinion. A good doctor won't just tell you that you have IBS and give up on you. The premise behind this treatment is to add bulk to the stool, which aids in both constipation and diarrhea. In some cases, IBS symptoms may be triggered by certain foods.