This particular topic comes up frequently on this forum, you can check the threads. The answer is always the same. A wall is load bearing if it carries more than its own weight. The ONLY way to tell is to examine the framing above. If any framing bears on the wall, including joists, beams, another wall, or diagonal elements, then the wall is load bearing. If your are not 100% confident in your ability to make this determination, you need to bring in a professional to do the determination and design the replacement header, assuming you want to modify the wall.