Responsive work fails when a design only exists at its happiest width. A long label, a short laptop window, a translated string, or 200% zoom tells you whether the layout has a real constraint or only a series of emergency overrides.
The practical test
Start with flexible containers, readable line lengths, and a navigation model that can become small without becoming hidden. Test the actual content, including its longest names, before declaring a breakpoint complete.
Related practice
Use this note as a prompt for a concrete review: identify the current public route, the source that owns it, the people it needs to serve, and the failure state it must survive.