The condition is easily diagnosed without a doctor (I'm not recommending anything...) but it's kind of one of those things one might want to be 100% certain because the changes to lifestyle are drastic. If you know a doctor whom you trust and think you have a problem with fructose, then a breath-test might be a possibility.
I only ask because I was diagnosed about 3 years ago and was misdiagnosed with IBS for 8 years. The whole experience showed me how much I should have not liked my doctors. The "formal" test for FM involves a hydrogen breath-test where they give the patient a solution of water and fructose to drink, then monitor the patient's breath for up to 6 hours (it was 4 hours in my case). The one people can do at home is go on a completely fructose-free diet, which is a difficult thing to do. After about a week, slowly introduce new food items that contain fructose or foodstuffs that break down into fructose. Keep a detailed journal of all portion sizes, types of food and research the fructose-to-glucose ratio. For example, I can eat a handful of raspberries or 2-3 strawberries once every few days but if I eat an apple, I've got to contend with stomach flu-like symptoms for the rest of the day.
My doctor, a gastroenterologist - who isn't my doctor any longer, led me on a wild goose chase for 3 months of medical test after medical test with the conclusion being a hydrogen test (they should have just given that to me first or told me to try the diet, which I was told about after the fact!). During that 3-month period, I lost about 40-50 pounds, had no idea why I was sick, and gained a whole new perspective of the medical complex. My FM story is pretty awful and totally falls under the TMI category, so I'm not going to go into details. I just wanted to know if there were any other FMers on the RPF and what some of their experiences were.