How to tell if the Google Update or 'Dance' has started -
1/ The links to major websites are different on each of Googles servers. To check, search for - link:www.yahoo.com - on www.google.com, www2.google.com and www3.google.com.
2/ The SERP's or Search Engine Result Positions are usually different on www, www2 and www3 for nearly the whole month THIS IS NOT AN UPDATE. This is what is refered to on WebmasterWorld as the 'Everflux'.
3/ Try not to create a new thread with anything like 'Google Update' or 'Has the Update started?'. This usually causes confusion amongst the members and also means extra work for the Forum Administrators as they have to keep deleting the threads.
4/ When a Google Update 'HAS' started you can guarantee that within minutes there will be a notice on the homepage of 'WebmasterWorld'. So when its near just keep your eye on the homepage and when you see it there you will know that its ON and its been verified by the people in the know. After that thats when the fun starts and you can check on your latest results on the www2/www3 Google servers!
5/ REPEAT only advise if the Google Update has started when you have double checked on at least 3 major websites 'Links Backs' on each of the google servers (www.google.com www2.google.com and www3.google.com).
6/ For a detailed history of then the monthly updates have happend take a look at Brett Tabke's 'Google Update History Chart'.