Define what "success" means in measurable terms before you start designing. After launch, close the loop by reviewing the metrics you set.
Look for a mix of quantitative and qualitative methods: task completion rates, time-on-task, error rates, SUS scores, NPS, qualitative feedback, support tickets, and A/B test results. Best candidates set success criteria before designing and follow up post-launch to validate assumptions.
Differentiates output-focused designers from outcome-focused ones. Candidates who never measure are flying blind. Those who obsess over metrics without qualitative context miss the full picture.