Prioritise your comments: mark blocking issues separately from suggestions and nitpicks so the author knows what must change.
Good answers cover: correctness, readability, test coverage, edge cases, security implications, and adherence to team standards. For feedback style: asking questions rather than dictating, separating nitpicks from blocking issues, acknowledging good work, and providing context for suggestions.
Reveals collaboration style and technical depth. Candidates who only look for style issues lack depth. Those who only look for bugs miss maintainability. The best reviewers balance both.