If you are using any theme based on framework which provide responsive UI then it became difficult to test it’s responsiveness.
Many developers are resize browser to size of mobile/tablet and test it’s UI due to which it became very tedious job.
Some developers do a different thing, they are creating a iframe and load site in this frame. This is better solution then resizing browser however this is not best solution.
The best solution is use
Responsive design mode provided by browsers.
For Firefox press ctrl + shift + M or Menu > Developer > Responsive design mode.
For Chrome press ctrl + shift + M or Press F12 > click on Toggle developer mode.