Removal of interior walls
The definition of a load bearing wall is one that supports more than its own weight. In order to determine if a wall is load bearing, you need to look at the framing above the wall to determine if any vertical load from above is transmitted downward through the wall. If you have plans for the building, you can determine which walls are load bearing by reading the plans. If you have no plans, you may be able to tell from exposed framing elements.
If the framing elements are not exposed, you are almost certainly going to need to open some holes to examine the framing. There are special cameras that work on the end of long, flexible, thin tubes that let you peer around through very small openings, and the pros often use these to do things like check for leaks, look for mold, or examine framing, through small openings that are easily patched.