Ideally, by looking a the number of times that I've experienced out-of-context problems in the past. You can optimize further by creating models that predict the base amount of novelty in your current environment - if you have reason to believe that your current environment is more unusual / novel than normal, increase your assigned "none of the above" proportionally. (And conversely, whenever evidence triggers the creation of a new top-level heading, that top-level heading's probability should get sliced out of the "none of the above", but the fact that you had to create a top-level heading should be used as evidence that you're in a novel environment, thus slightly increasing ALL "none of the above" categories. If you're using hard-coded heuristics instead of actually computing probability tables, this might come out as a form of hypervigilance and/or curiosity triggered by novel stimulus.)