thanks so much for the information. When we have flown Sansa or Taca in Costa Rica they seemed to have strict weight limits. They would weigh your bags on meat hooks and then ask for your own weight. I believe the weight limit for luggage was 25 or 30 pounds
I've certainly never heard of people's own weights being requested, though logically perhaps they should. I daresay they trust to the pilot having a good eye when he looks at who and what are boarding his aircraft.
I've actually been in a Caravan (the normal aircraft here) when all five other passengers were seated as far back as they could go. I was surprised the pilot didn't ask them to move forward. There seemed to be no takeoff or landing issues, so I deduce that balance is not a critical matter for these aircraft.
By the By, on a humorous note, The Belizean Port Authority determines capacity by 20" of seat width. For me this always brings up a visual of the captain checking passenger overload with a tape measure.