Try keeping a food diary of everything you eat and drink, medicine you take, and when you get sick and with what problem. After two or three weeks you may start to notice a pattern. If you have suspicions, then try eliminating those things from your diet to see if your symptoms improve.
I will warn you of this: the lactose pills don't work for everyone so you can't rule out lactose intolerance until you eliminate all dairy with lactose in it (i.e. only drink the lactose-free milk) and see if that helps. Also, there are two levels of dairy problems: the inability to digest lactose and the inabilty to digest or even actual allergy to all forms of dairy, including the milk protein casein, which is a common ingredient in pre-packaged foods.
Some other things you might want to try eliminating are gluten (the main allergen in wheat), meat (starting with red), artifical sweeteners, and all forms of real sugar (including fructose from fruit). You can also try an all-organic, dye- and preservative-free diet. Some people are hyper-sensitive to dyes, preservatives and other chemicals. This is especially likely if someone else in your family has a similar problem.
You may want to try a different fiber supplement; certain ones seem to work better for certain people than others. You may also want to try a month's regimen of probtiocis, like Digestive Advantage; some people have gotten help from it.
While you probably do have IBS, there are some tests that doctors can do to see what may be causing your IBS. You should try and see a G.I. and at least have a breathalyzer test to see if you have a bacterial overgrowth. Some early studies are starting to point to that being a very common source of GI problems. It seems to be very hard to treat, but antibiotics and probiotics can help. Also, if you see a doctor, you can, as vetchick suggests, try an anti-spasmodic to help relieve the cramping and the multiple trips to the bathroom. They are a lot like fiber--some work for some people and some don't. The hyoscyamine that vetchick says works for her doesn't do a thing for me; I might as well drink water. But there are a lot of them out there, so if you get one that doesn't work, go back to your doctor and ask to try another one and keep on until you find one that does work.