bluntforce - 6:59 am on Feb 9, 2013 (gmt 0)
You look at your inbound links.
If there's not a date defined Panda or Penguin correlation, but you have a long slow decline? Perhaps it's not your site, but issues with the sites who link to you.
How many new quality inbound links have you generated in the last month? 3 months? How many low quality inbound links have you generated?
No new links? Think about it.