A load bearing wall that is designed to be there for support of something above it will usually run the length of the structure and perpendicular to the ceiling joists usually close to the center line and there will be headers/beams over any openings. Just being perpendicular to the ceiling joists doesn't necessarily make it load bearing, by design, as most bedroom, kitchen, and bathroom walls can be framed up every which way according to the floor plan but aren't necessarily designed as such for load bearing. Look to see if any partial wall has a header/beam extending on to another wall which would indicate that it is load bearing. Look to see if the joists/trusses above the wall use that wall as an end point where overlapping joists then extend towards the outer walls which then indicates a load bearing wall.
Full length joists and trusses may or may not be self supporting and their size should be compared to appropriate span and load bearing tables depending upon the load they are to carry and an architect/enginer involved in house planning or the floor/roofing truss builder at your local lumber yard would know this. The use of load bearing walls will allow builders to use a smaller joist size and avoid big beams. Trusses for cathedral ceilings are designed to not need a load bearing wall under them for example.
Basement floors and concrete slabs will have footings poured under that portion of the concrete that is designed to be load bearing which is part of the initial house design before construction even starts. A typical rectangular ranch home will probably have a single load bearing wall run down the length of the house within a few feet of the center line.