You can literally tell by looking whether a container ship is empty or full...that’s how easy it is to prove/disprove what you’re saying.
There are a dozen other less direct ways to tell also (fuel consumption, transit speeds, shipping company financials, etc)...not to mention the 1000s of people working crews and ports.
Get a grip son...
Ever put a container on a scale, man? They are damn heavy, with or without cargo. And all of the ships out there are fully loaded with containers.
It’s not as easy as you may think.