How do airlines without assigned seating verify cargo balance? How to approximate the mass of passengers is well explained, but balance of the aircraft is concerned more with where they sit. Other answers have hinted that assigned seating locations are one method for calculating loadsheet balance, but this is not always an option.

Certain passenger airlines (such as Southwest) do not assign seats, allowing passengers to select whatever seat they like during boarding. Since passengers must be accounted for in load balance calculations, how do these airlines know how passenger mass is distributed? Are there strategies besides knowing exact locations to determine whether cargo balance will be within the envelope? Do they simply have flight attendants record where passengers are seated?
The cabin crew simply counts the seated passengers (or alternatively empty seats) by zone (a block of rows), writes it down and hands the slip to the flight crew. It is common to do this on commuter planes even with assigned seating since passenger balance matters more on small aircraft and people may move.

As pointed out by SKYbrary:


  In practice, low cost airlines, which routinely offer free seating, also operate the majority of their flights with a high load factor, so that identifying any unoccupied seats will be a fairly simple matter.