This question comes up frequently: If you know you react to a food within a group (eg wheat/fructans) can you assume that you will react to all foods within that group.. in other words could you react to some fructans but not others?
Good question. Remember with FODMAPs we are trying to assess tolerance to a type of carb (ex. fructans) in our food rather than out tolerance to individual foods. If you do an organized challenge process, the outcome might be that you learn you do not tolerate fructans in general very well. But you cannot assume that just because you react to a single food in a group that you will react to others.
Here are a few reasons why: You might be allergic to that food; you might have celiac disease or non-celiac gluten intolerance(in the wheat example you gave); other foods in the same group might have much less fructans per serving than the one that bothers you; your portion size of other foods in that group might be much smaller, the specific type of fructans in the food might be more or less rapidly fermentable than others, etc.
It's important not to overgeneralize, because its a nutritional mistake to unecessarily restrict your diet.