The key indicator I use is webmaster/SEO chatter. I check the chatter at WebmasterWorld and dozens of other forums and the chatter has picked up yesterday. Martin Ice Web, who is a Senior Member at WebmasterWorld but is based in Germany, is the loudest on claiming updates today. There are many who agree and see major changes and there are many threads in the Google forums with individual complaints.
That being said, one of the tools I rarely show you, because it often doesn't match the other tools, is the DigitalPoint Keyword Tracker averages changes. It is something fairly new added to the forums sidebar and this reports changes of actual rankings for hundreds of thousands of sites and I'd say millions of keywords. It is based off what webmasters enter into the tracking tools.
It showed a bit of a change on the 17th but on the 19th, it really skyrocketed, like the forums did.
I've emailed Google yesterday to find out if something is going on specific with rankings but I have yet to hear back.
It can be a refresh of Panda or something else but I have no confirmation from Google.