I'm not sure how one would know if they IBS too. Isn't the diagnosis of IBS basically when they have ruled out all other diseases? If they've found Crohn's, I'd have a hard time believing it's IBS too. Unless they can tell me 100% for certain I don't have a flare up going on somewhere in that LONG small intestines of mine (that they can't see all of).
The whole IBS diagnosis annoys me though, because that was the first thing most doctors jumped to before I got my Crohn's diagnosis. To me it's become a staple of "I have no clue what you really have, so we'll just call it IBS". Or in a Crohn's patient perhaps "Can't see an area of flare up, so must be IBS!" Maybe I'm wrong, but it just seems like they are quick to label everything as IBS, and many of us go without treatment we need.
Most Crohn's patients will tell you that even in remission, that doesn't mean they are symptom free. I don't know, when I have my bad days I don't look to thinking it's IBS. I just believe it's the Crohn's waving "hi".