How to know whether the website is responsive or not?

If you want to know whether a website is responsive or not, then the easiest way is to open that site in the browser bar and then try to increase or decrease the size of the browser window. If the components of the website shift and adjust according to the window size, then understand that the site is responsive.

You can also use the developer tools available in the browser or any online tool like isresponsive.com or respondsinator.com for this work.