The easiest way would be to put the browser in Window mode (not Fullscreen) and just drag to resize and shrink it. If the website adjusts to the window size, then the website is responsive. You can also check different screen sizes on different devices, like tablets or mobile phones.
Alternatively you can use an online checker, there are plenty of them:
The best and easy way to check the website is responsive or not is to resize the window size as said above by the @Rado_Ch . another you can check this by using the input to the websites.