As the word responsive suggests, this type of testing is used to check the appearance of a website with RWD(Responsive Web Design). This design helps a website to render the content in a flexible yet appropriate manner on the basis of the devices & OS on which it gets summoned.
I use LambdaTest to test the responsiveness of my website. It is a free of cost tool and helps me to check responsive testing along 44 different mobile devices in a single test session.
It also helps me to perform cross browser testing across every major browser, absent cost.
Another common way to check how your RWD is adjusting to different screen sizes is to drag the any corner of the browser window and then slide the corner in order to minimize.