How do I determine if a wall is load bearing?
OK normally load bearing walls will have 2 plates on top.  But having 2 plates means nothing.  

People frame however they learned to or want.  Checking door header also means nothing.  Some people flip all door headers - doesn't make the wall load bearing.  You can never get into the head of the guy who framed your house.  I worked for a construction company in college... the owner taught everyone to basically frame every wall like it was load bearing - 2 top plates and headers flipped was a minimum.  I asked him one day why we were doing this and he basically said - Well it might cost an extra $10 on some walls but we never have to worry or redo anything.  

You told me this wall goes in same direction as attic joists...  That is a good sign.  You need to get into the attic and see what it is attached to.  If it isn't supporting a joist it isn't load bearing.  If this wall is just floating between 2 joists - even with a few 2x4s for cross bracing - it is not load bearing.  

However if there is structural bracing in the attic it is/could be load bearing.  Or if it is directly under a joist.  

You have two choices to figure this out.  Get up in the attic or tear down enough drywall to figure out what you have.  If you are a gambler and want this wall down just tear out the drywall - you will have to remove it to the joist on each side to redrywall anyway.