How would you identify a load bearing wall?

This depends on the type of construction.
My guess is that you want to either take down or open up a wall and that's why you're asking this.
You really should get a pro to look at it before you go knocking anything down,
because any wall could potentially be load bearing.

Go down into the basement and look up at the ceiling, you'll see the joists.
These are heavy boards that are 1 1/2" to 2" wide and 8" to 12 " tall.
Unless the house is really tiny, they will be attached to the main beam that's got some posts or a wall supporting it.
If the wall you want to mess with runs in the same direction as the main beam, it may be load bearing, especially if there's a flight of stairs attached to it.
If it sits directly on top of the main beam, it's definitly load bearing and you'll need to add more structural elements to make up for what ever you're planning to take out.
If the wall runs in the direction of the joists, it mat be a curtain wall. (not load bearing).
Try and find the joist directly under the wall. Sometimes they're doubled up under a curtain wall, but if they're 3 or more joists under the wall, or there's stairs attached to it, don't mess with it.