You can check if your images are being deferred by inspecting the console. If you are using Chrome, right click and press inspect.

Go to the network tab and then refresh the page (I always hold down Ctrl + F5 which is a hard refresh)

You will see the following:

As you can see above the images [1] are being loaded after the initial page load. The red line shows the initial page load [2]. If the images are on the left of this line, it is NOT working.