Name multiple quantitative and qualitative signals. No single metric proves PMF - show you triangulate across retention, NPS, organic growth, and qualitative feedback.
Look for multiple signals: retention curves flattening, organic growth, the Sean Ellis survey (40%+ "very disappointed"), strong engagement metrics, low churn, and qualitative signals like users recommending the product. Best candidates discuss the difference between early traction and true PMF, and how they iterate toward it.
Core PM concept. Candidates who only cite the Sean Ellis survey lack depth. Those who discuss the journey toward PMF and the iterating process demonstrate real experience building products.