This step is optional. But it is one I wish I had done to catch the lack of copper layer on my boards.

I don't find the JLCPCB Gerber viewing capability as flexible as I found KiCad. So if you'd like "a second opinion", look for a 3rd party browser-based Gerber file viewer.

Whichever viewer you chose, it will not show the extra color printing on the silkscreen. Just overlook this detail and focus on the layers.

For a design printing color on the silkscreen not showing copper or FR4, you can likely skip this step.

If you are looking to view the copper and lack of silkscreen, I used the PCBWay viewer here.

The key to a viewer is the ability to toggle layers to ensure the design for various PCB elements is to your board layout. If something is missing or amiss, you can tell.

If all looks ok, it is time to order!