We will know it is conscious based on our emotional connection with it. Shortly after that, we will build machines much more intelligent than humans, because intelligent machines will help with their own science and engineering. And the knowledge gap that has been shrinking over the centuries will start to grow. Not in the sense that scientific knowledge will shrink, but in the sense that people will have less understanding of their world because of their intimate relationship with a mind beyond their comprehension. We will understand the machine's mind about as much as our pets understand ours.
Some people ask whether machines can ever be conscious.
Many biologists believe that larger brains gave early African hominids a selective advantage because they enabled the hominids to maintain social relationships with groups of 150-200 others via language and other new abilities, and working in larger groups was an advantage. This defined the distinction between human and animal consciousness. The Internet is reaching deeply into our lives, and with ubiquitous computing will reach into every significant human-made object. Machine intelligence will evolve in the servers for all these objects, through which it will maintain constant contact with us. Metcalf's Law says that the value of a network is proportional to the square of the number of people connected to the network, and this will apply to intelligent servers.