Using Hash Value (track_changes) To Detect Source Data Changes in Migrate for Drupal 7

When you import data with Migrate (http://drupal.org/project/migrate) it is nice to have ability to update already imported data if source has changed. To track updated data Migrate uses highwater marks (more on this: http://drupal.org/node/1223936). Highwater mark is a column of a source data which has timestamp of the last data change. However, the problem with highwater mark is that you don't always have such "time tracking" enabled on source's page.