Of course, you could try arguing that I just dreamt that I was making conscious decisions, but that's just splitting hairs.

That's exactly what I was intimating. Some believe that freewill is a similar illusion. There is no clear way to know, except that within the restricted arena of your dreams, assuming you're an honest person, then we can ask you to dream a certain thing in advance and see if you can.

That doesn't rule out the dream being a created memory that doesn't occur until you come to "remember" it however.