Acceptance criteria should be testable and specific. "Page loads fast" is vague; "Page loads in under 2 seconds on 3G" is verifiable.
Good answers cover the user story format (As a... I want... So that...) but also go beyond it: including context, success metrics, edge cases, and clear acceptance criteria. Best candidates discuss the conversation around the story being more important than the written artefact, and mention INVEST criteria.
Fundamental PM skill. Red flag: writing requirements in isolation without talking to engineering. Good sign: treating stories as conversation starters with clear, testable acceptance criteria.