First, to be able to test our website's responsiveness, there is a tool available in Chrome that is very useful. To access it, you can go to View | Developer | Developer Tools:
To use this tool, click on the second icon on the top left
Now, you can select any device you want to test with, as follows:
You can also see that the code of the entire page is displayed on the right-hand side:
The Google Chrome Inspector
This is very useful, because it allows you to try different CSS properties before putting in your code or to check whether your CSS works. You can have a quick try and play around with it.