from my experience it is diagnosed by symptoms, which are stomach ache/pain, often bloating and diarrhea . i think it is also causes excessive pressure in the j pouch, which is not relieved by trying to evacuate. there are some tests. i know there is one for lactose. honesty sharon's suggestion of eating ice cream is easier than going to hospital for that test.
if you go on the lowFODMAP diet you will start getting relief within the week and certainly after two weeks. the relief is another indicator that you've got it. estimated that 15% of the population has this problem!!
here are some links:
http://www.med.monash.edu/cecs/gastro/fodmap/ -- get their app!
my doc said i wouldn't be able to figure it out without going to a qualified dietitian. that was just the challenge i needed, and i did figure it out. finally went to a dietitian and she was of some help. it is tricky. for ex, if you take a probiotic the prebiotic in it--FOS--is a FODMAP, so either stop the probiotic or get one without it. i can't even take a probiotic. read every label to make certain no hint of a FODMAP is it it.
once on the diet i entered a religious domain, felt so much better and that encourages me to stick to it.
good luck! check in with your progress please. janet