IBS is a diagnosis of exclusion, which means they can't test for it at all. If you have digestive symptoms and all your tests come back negative, doctors diagnose IBS. In my opinion doctors diagnose IBS when they don't know what the problem is, or when they haven't bothered doing enough tests.
Exactly how many and which tests you need to have to ensure you don't have anything other than IBS causing your symptoms is very uncertain. Some doctors will diagnose it without doing any tests at all just based on your symptoms.
What symptoms are you getting, if you don't mind my asking? If you lose weight or pass blood, those are big signs that it's something more serious.
Medications aim to address the symptoms, as the cause of IBS is completely unknown and can't be cured. If you have diarrhoea, medications like Immodium and Lomotil can help. For constipation, stool softeners can help (these are safer than stimulant laxatives, especially if you need to use them long term). There are various other meds for symptoms such as pain and bloating, but I'm not so familar with these. I've read that diet can help manage IBS symptoms as well, though exactly which dietary changes might help you is something you have to figure out by trial and error (just like with Crohn's), because it's very individual.
Just a quick warning: some doctors don't take IBS very seriously. Make sure you find a doctor who understands the effects symptoms can have on your quality of life, even if there's no serious threat to your health. If your symptoms get worse or you devlop new ones, make sure you or your doctors don't jump too quickly to an IBS diagnosis.
It's also my personal opinion that a lot of different conditions get diagnosed (or misdiagnosed) as IBS. There's an awful lot of uncertainty surrounding it. Because of this, and because of doctors' tendency to treat it as no big deal and/or as a psychological or lifestyle problem, I think it's a very unhelpful diagnosis.
So if I were you I'd only accept an IBS diagnosis (if that's what your doctor decides it is) if your symptoms really are mild enough to not cause you any real problems.
I hope your symptoms remain mild - or even better clear up on their own.