Depending on the make and model, some floors can be inspected directly inside cabinets around the perimeter of the shell where leaks
and rot are likeliest. Look for discoloration, smell for mildew, tap and probe for softness. I agree it is one of the most critical parts of the pre-purchase inspection. I am always just a bit suspicious of brand-new finish flooring.
I’ve never used a moisture meter. Would it be able to detect moisture in the core of floors that are fully encapsulated in fiberglass? Those are less susceptible, but they are also the hardest to inspect and trickiest to repair.