1. What business problem are we solving, and how will we know we succeeded?
  2. Which facts have been confirmed, and which are still assumptions?
  3. Which quality concerns — availability, performance, security, data and cost — change the design?
  4. Which decisions are reversible, and which require an ADR and explicit acceptance of risk?
  5. How will we test the most important assumption before implementation becomes expensive?

You can use this checklist to review an HLD. The ADR guide explains how to record a consequential decision.