Now, you can turn on Device Mode right next to the "inspect element" icon. Once enabled, it automatically emulates a mobile viewport, along with complete emulation of touch events. You can test various viewport sizes easily with the large drag handles instead of resizing the whole browser window. You can select from popular device presets to automatically set viewport, touch, user agent and screen density settings in one fell swoop.
Every media query gets represented visually, so you can correlate your breakpoints with the viewport. Clicking one resizes the viewport, making it easier to iterate on your associated styles. If you want to edit the media queries themselves, right-click and jump to the line of CSS where it's defined.