Desktop and mobile pages often fail in different places. Start by checking whether the same primary action is visible, understandable, and reachable in both screen sizes.
Look for navigation changes, hidden pricing details, cramped plan cards, forms that become harder to complete, and content that shifts below the point where users make decisions.
When you compare captures side by side, the conversation gets much more concrete. Instead of debating the idea of responsiveness, the team can point to the exact state that needs work.