> how would we know if a machine was conscious?

1) Knock it on the head, and see if it becomes unconsious.

2) More philosophically: Related questions* have been on the books since at least the days of Descartes, one way and another, but I can't remember the details. But methinks the Moody Blues were on to something with:

I think I am .... therefore I am .... I think.

* How can one human tell that another is conscious?