Is it possible to use features or some other technique to compare two sites?
If you are using Drupal 8, yes you can. But if you are using Drupal 7 I doubt you can yet do it. There is a backport of Configuration Manager to Drupal 7, which might make it possible.

The reason I said you can't do it in Drupal 7 is that the settings are saved in database and it is very difficult to make a comparison of the databases for settings. So if you have already made the changes and don't have a log, I would suggest exporting individual features using Features Suit of Modules. 

You can checkout my presentation http://www.slideshare.net/gokulnk/features-15841090 for related Features modules which will make your work easier.

As a suggestion next time you want to keep a journal of your configuration changes please use Journal Module which should serve you well to keep track and document your changes.