There are a number of tools available to see how a website design adapts at various device sizes – I have normally used Responsiantor as my go to tool in the past.
However, a handy one I came across recently was the built in responsive view in Safari. First up – just open any website in Safari:
Now just hit: Control + ⌘ + R and you will enter Responsive Design Mode in the Safari browser:
You can then preview your webpages for various screen sizes, orientations, and resolutions. To exit Responsive Design Mode you can just hit Control + ⌘ + R again.
Read more about the feature on the Apple site here.