How Do You Identify the Cause of a Penalty if only google algorithm update?

You correlate your website traffic stats to known dates for Google Panda, Penguin etc. updates

If your website affected by Google algorithm update, you can see a Sudden or Steady Drop in Rank.
Google updated its algorithm and many of your previous backlinks were either de-indexed or discounted. Since the total value of your backlinks dropped, your ranking dropped.
Un-natural links have become increasingly important over the last few Google updates.