How to detect which WebPart zone is empty?

Question

I have 2 columns and 2 row div structure in my custom page layout of Publishing type. I have kept 4 WebPart zones in each boxes made by div structure.

Now my requirement is, the box with no WebPart added should not be displayed on the page in display mode.

But this is not working here.

So, my question is:

How do I detect which WebPart zone is empty?

Note: I am having only SharePoint designer to work with as its SharePoint Online.