This question is easy to answer, although getting the needed information can sometimes be difficult. It typically means getting up in the attic and possibly moving some insulation to expose the ceiling joists.
Essentially, a load bearing wall is a wall that parts of the house framing rests. This usually means ceiling joists and roof bracing. So, it requires some exploration of the attic of the home. This exploration is best done during mild weather or early in the morning. The attic gets very hot during the day and takes several hours to cool down. Some insulation can be uncomfortable to handle, so long sleeves and gloves are suggested. You will need to study and somewhat memorize the layout of the wall in question relative to a point of reference, such as the attic stair or possibly a fireplace chimney. Take some measurements and take note of recessed lights and air conditioning registers. These will help you locate the wall once you are in the attic. Stay on the wooden platforms constructed in the attic or only step on the ceiling joists if you must do so. You will be looking for the top plate of the wall, which will be a piece of wood approximately 3 ½ inches wide immediately below the ceiling joists, with sheetrock on either side of it. The sheetrock is the ceiling of the rooms below. The ceiling joists will be larger boards standing on edge, with each end resting on the top of the walls.
Once you locate the wall in question check the entire length of the wall, as attic framing can be somewhat inconsistent. If the top plate is parallel to the ceiling joists and does not have any roof braces (boards) resting on it then the wall is not a load bearing wall. If the ceiling joists or any roof braces are resting on the top plate, then the wall is a load bearing wall. This question is typically associated with a desire to remove the wall. If it is load bearing, that does not mean it cannot be removed – the job is just more difficult. In such a case it is strongly advised that a competent professional be contacted to determine the proper course of action. Likewise, answering this question on a two-story home can be much more challenging and a professional should be engaged.