The first place to look for the question or problem in a scientific paper is the abstract. An abstract is a short summary of the scientific paper. Abstracts are usually a single paragraph found on the first page, placed below the list of authors and their affiliations. The abstract should include sentences stating the problem or question, the methods, results and conclusions of the scientific study.
However, in some cases the problem or question may not be clear to you after reading the abstract. In this case, you should dive into the other sections of the paper. The introduction or background section usually follows the abstract. This section is used to give the reader a brief introduction to the problem, and may include an explanation of widely-held beliefs or recent work in the area.
If you are still unsure, you can also check the conclusions section. By reading the author's conclusions you may be able to deduce the original problem or question they were investigating.