How can I determine if a wall is load bearing?
Load bearing walls typically run perpendicular to the floor and ceiling joists. So if you can figure out which direction those run in, any wall perpendicular to that has a good change of being load-bearing. 

If there is a crawlspace you can access, that's an easy way to determine floor-joist direction. You can also spot the support pillars/poles underneath as well. Any wall above a support pillar is also likely load bearing. 

Other than that, you need to bring someone in that can help you figure it out (contractor/engineer). 

Note that if you are open to the idea of keeping 'parts' of the walls, that's helpful, as said parts can then house support columns for beams that span the section of the wall you took out. How large those beams need to be goes back to needing a structural engineer for help.