How to determine if dataset is a suitable representation of the context?
Before collecting the data, domain knowledge is used to determine plausible variations the task may present. It is not necessary to capture in data all the variations experts may point out (for example, some may be syntheticaly constructed). But domain knowledge indeed dictates the variations a certain task may have to take account for.
That being said, one then gathers the data trying to match plausible variations in the percent that domain experts dictate. Again some may even be syntheticaly constructed a-posteriori. Then we have a representative dataset.
In case the dataset proves inadequate, this means domain knowledge needs to be updated with new information about the task.