Each regulatory authority seems to have its own idea. In Europe, EU OPS 1.620 defines pax mass according to type of flight and size of aircraft. See page 125-126 of
http://eur-lex.europa.eu/LexUriServ/...1:0238:EN:PDFd
for more detail.
When the CAA decided these things in the UK, the weights/masses were raised every few years as the result of surveys of real pax at check-in. During my airline years, the figures we used seemed to be fairly realistic in that the aircraft performance was pretty much in line with expectations. Without an on-board aircraft weighing system, I think there is no other way of the crew being aware of true mass - and of course only gross errors would show up that way.