Assessing whether a wall is load-bearing is easier with access to the attic, but it can be done without if the building has a basement. Check the basement for support posts, girders and beams that stretch the length of the room. Walls that are above or perpendicular to these support structures are likely to be load-bearing.
A wall is considered load-bearing if it provides structural support, which means that the wall plays a role in supporting a structure such as a floor or a roof above it. For this reason it is vital to identify load-bearing walls before beginning any remodeling work. Knocking down or otherwise changing a load-bearing wall without first securing its load with the use of temporary supports causes serious structural damage to a building.
It is also important to know that making changes to walls sometimes changes which walls are load-bearing. Remodeling and new construction may add stress to walls that previously did not bear significant loads. For example, the sudden addition of a room to an attic that was previously empty changes the way the walls distribute weight. Pay close attention to what is above each wall.
All external walls are load-bearing because they support the roof, but identifying interior load-bearing walls is much harder. If there is any doubt about whether a wall is load-bearing, consult a professional contractor or building inspector. The danger and damage certain to be caused by misjudgment far outweighs the cost of hiring a professional.