For the other parts of New Fantasyland, in mid-October 2012 they opened up Mermaid, Gaston's, and Belle to the public...grand opening ceremony was early December 2012. No announcement or anything that the rides would be open. People just arrived at the parks and saw the walkway back there was not blocked and CMs were inviting people back.
I was there for the second through ninth day of the general public soft openings, and some mornings/times the area was closed off. I think as the official date got closer, the area might have been closed for an entire day or so, and it was also closed at times for the scheduled previews for AP, DVC , and CMs.
Note that these general public soft openings happened *before* the previously scheduled invitation-only (DVC, AP, etc.) previews. So one sign that soft openings could be happening soon is if they are scheduling these special previews.
Also, the Belle experience had quite a few test/preview sessions leak out several months before the soft openings. A few time the general public was invited into one of these but there were right-place-right-time situations that you couldn't line up for (unless you're a Disney blogger and a CM tips you off and you decide to stand by this construction door for a few hours one day).