It is like @CanIUse but for the browser devtools, created & curated by @pankajparashar. Get weekly tips & tricks for your favorite browser devtools by subscribing to the newsletter!
In the Elements tab, select the node with label "flex" from the DOM tree.
From the right sidebar, select "Layout", and scroll down to the Flexbox section, that has few page overlay options to help visualize and debug flexbox containers.