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?