Design the error state first. If your error handling is graceful and helpful, the happy path will take care of itself. Empty states and loading states deserve the same design attention.
Look for: empty states, error messages, loading states, partial data, permissions variations, first-time versus returning user experiences. Strong candidates design the unhappy path with as much care as the happy path. Best candidates mention that edge cases often reveal the most about a product's quality.
Separates thorough designers from surface-level ones. Candidates who only design the happy path will create products that frustrate users at the margins. Ask to see edge case designs in their portfolio.