How would you determine whether a SQL index should be added?

Start by examining the query’s execution plan for table scans. Identify columns used in WHERE, JOIN, ORDER BY, or GROUP BY clauses. If a column has high selectivity and is frequently queried, an index is beneficial. Consider composite indexes for multi-column predicates. Also assess write impact: indexes add overhead to inserts/updates. Use statistics and test performance before and after adding the index to ensure it actually improves query speed.