Sarita's right in that all three of these things have overlapping symptoms and it can be very hard to figure out what's wrong.
Crohns: Weight loss, constant diarrhea (although constipation is, very occassionally, a symptom), pain, problems in other systems--such as the kidneys, family history of ANY type of auto-immune disease. Crohn's symptoms are typically--although not always--worse than the symptoms of the other two disorders. A colonoscopy is the best (although not fool-proof) way to find it. Blood tests, as Sarita mentioned, may find the inflammation in the colon that way.
GB-related problems: heartburn, yellow or green diarrhea (although a small number of people get constipation instead), burning poops, very obvious connection between eating and having stomach/intestinal problems. May have some pain, although this is usually not a primary symptom and is rarely present without very bad D at the same time. Gas and bloating to some extent.
IBS: pain, D or C, gas, bloating, connection between eating and having inestinal problems.
Basically, you need to have tests to rule out Crohn's. Then that leaves you IBS or GB problems. If you have bile in your stools, then it's almost certainly a GB problem. If you take Welchol or Questran and get better, than the post-GB was your problem. If you don't really get better, then IBS may very well be your diagnosis.