Drag the width of the browser to the preferred breakpont. if you have a layout at a very narrow breakpoint, make your browser window as narrow as possible then use the zoom keystroke (command =) to force the display of the narrow breakpoint.
If you are using Safari, use Safari's Preferences > Advanced to enable the Develop menu. This will cause the Develop menu to appear in the menu bar. Select "Enter responsive design mode" and you can click on the various device sizes to instantly jump between breakpoints.