I think you will find it is the regulator that sets the "standard weights" of pax not the aircraft manufacturer.
So it wont even take a smart pilot walking past his passengers to notice if many/most or all pax are over weight.
Lets say 90% appear to be a bit fat as the pilot walked down the isle - no problem here it is perfectly legal. Because it has been approved by the regulator and data had been given to average weights on certain things - this case passengers.
Now I know many companies that run extensions, on engines ( I know it not a weight thing) that are approved because a company has developed a method that has been deemed acceptable (this can be trend testing and overhaul limit checks), The regulator also has approved methods to test colour blind pilots - but that's a different story.
Now it is certainly possible to have a approved system in place to handle small overweight conditions (so may just need to be recorded) - the over weight landing checks for instance depend on by how much over weight was the landing, a very small amount requires a very small inspection, a large amount is a different story.
FYI I have worked FAA, EASA, CASA and a few of the CAA's