This is a question which as a result of a Cessna Caravan accident in Canada has been much debated in recent times. In that accident the calculated weight of the pax turned out to be 567lbs less than the actual weight. Significant when one considers that in a small aircraft like the Caravan the pax weight accounts for about 22% of the total compared to, say, 7% in a 747.
However, to answer the question... No secret weighing! Average weights are generally used (this is actually quite complicated so this answer is a generalisation and applies to large aircraft) The 'standard weight' varies dependent on state of registration but typically around 190lbs in summer and 195lbs in winter.