The most sure way to check if a wall is load-bearing is to read the original construction blueprints. If those are not available, however, there are several alternate ways to determine if a wall bears a load.
Any exterior wall that sits on top of the foundation is load-bearing. Walls that sit on top of the foundation walls bear the weight of the roof. Walls on the first and second floors that are near the center of the house and parallel above the center basement beam are usually load-bearing. Walls that are perpendicular to floor joists or end in large columns or posts are also usually load-bearing. If unsure, bear in mind that the cost of contacting a professional for an inspection is significantly less than the cost of repairing damage.