I've been a pilot most of my life. On larger planes with more people, an assumed average weight is used. That number can change for certain occasions. For example, when we flew a pro football team the number was much higher than a normal passenger flight. On small planes, in addition to the total weight, the center of gravity (cg) is important and must be within certain limits. Also, I have learned to judge weight by eye because some people will lie to you. If things are really close, the answer is to use a scale.
Lew