A broad approach is to know an issue through extensive literature review. Very often, published papers introduce their work by mentioning the gaps existing in knowledge and what the papers are contributing — and what should be done next. With every paper you read, your sense of awareness of existing gaps in knowledge increases.
Ultimately, you’d naturally need to draw up a checklist to better appreciate the ‘what is’, and ‘what should be.‘ This is wHy the most immediate task before and during a research proposal is literature review.
Others follow a more structured approach — but still, literature review remains central.