Load bearing walls are constructed perpendicular to ceiling joists. The ceiling joists rest on a load bearing wall. In your basement(if you have one) there will be iron or wood posts that run in a line from wall to wall. That pattern will continue up through the house.
Ron

Cool. Thanks again Ron. I'll have to check the basement out when I get in there. Cool. Thanks for that tip.

What if you don't have a basement? My bathroom is downstairs and I'm trying to find out if there is a load bearing wall, but without opening it all up, how can I find out close enough?

If your ceiling joists span exterior wall to wall, perpindicular walls are not load bearing. If the joists are spliced and land on that wall,they are still not necessarilly bearing, but you dont want to interrupt the splice.If you have access to your attic, you can see if there is any roof bracing landing on that particular wall,or any structural element indicatiating a load bearing wall.If no access, as in mariposatraicionera(phew!)'s case, you will have to cut out an inspection hole in the area of the wall enough to get a look to see if their is bracing, or any other element landing on the wall making it load bearing.

Properly supported bearings start from the foundation and travel on up. On a crawl space or basement,as ron stated,It starts at the footing or pier and is supported up through the floor,then posts or a wall, on up to the roof framing that is braced off of that. All supported down below at the foundation level. This is why trusses and engineered products such as i-joists have come in to play, because they span uninterrupted, making interior walls partitions rather than load bearing.

mariposatraicionera, don't you have access to your attic? Do you have a pitched roof? Usually the ridge line of a roof will indicate the orientation of the load bearing wall(s). But not always. Sometimes the joists are switched in direction from the first and second floor. As in my house. Basement/first floor rafters run side to side with the two metal I beams running front to back. Attic floor joists run front to back to support the roof joist orientation.
If you're unsure, call in someone whos knows.
Ron