Always start with the questions the data must answer, not the data you have. The business question determines the grain, dimensions, and facts.
Look for: understanding the business questions first, identifying required granularity, choosing appropriate schemas (star, snowflake), defining dimensions and facts, considering query patterns, and planning for scalability. Best candidates discuss the importance of getting the grain right and how modelling decisions affect downstream analysis.
Senior analyst skill. Candidates who jump into building tables without understanding requirements create models that need constant rework. Ask: "How do you handle slowly changing dimensions?"