The plywood on mine has a spray painted label that says the thickness.
When I use a step stool to crawl inside the storage compartment, I can see the label on the underside of the floor. A flashlight helps.
I forget what thickness mine is, but maybe 19/32".
If you can find a label, that is the best and easiest way to know the thickness. You can also measure the depth of the crack with your choice of measuring device.
Chris and Pat
2016 Ram 3500 Cummins Aisin Dually, 4.10 axle, Auto Flex Air Ride Rear Suspension, Pace Edwards JackRabbit tonno, Transfer Flow aux fuel tank, B&W RVK3600 hitch
2017 Cedar Creek 36CKTS, MORryde IS, Disc Brakes, 17.5"