how can you tell what is a low bearing wall,and wich walls hold up the house

Bearing walls generally run perpendicular to the rafter/ceiling joist. If you have access to the attic you can see which way the rafter/ceiling joist run and posssibly where the wall is at in relationship to those.

You mean a "load" bearing wall. Telling what is one of those can be tricky. Looking at the framing can tell you what is load bearing, but that's probably hidden by drywall. So you may have to remove some drywall.
You can also tell by the roof framing, maybe.
You can also tell by any support structures you may have in a basement, if you have one. On a slab floor, you can't tell just by looking at it.
Or you can tell by framing supports in a crawlspace.

But you have to have knowledge of framing and roofing and etc etc to know what you're looking it. If it's a short wall with no real heavy pieces of lumber, then it's probably not a support wall. But you should pay someone knowledgeable to come and look at what you have in mind. Trying to get free help may be more costly than its worth.

This message has been edited. Last edited by: Re-mdlr,

there ARE houses where there are a number of little stub walls here and there, where there is no notable load-bearing wall.

in which case, those little wall-lets ARE the load bearing wall, and there had better be extra joist and truss construction above and below to move the weight onto those stubs.

not common residentially, but when it's there, they spent a bunch more money on structure than if they had run a visible header or a standard load bearing wall down the middle.

where and what do headers do above a new french doors i want to install when i cut out a old window