How can I show only modified fields on item

I have a list with 60 fields in SharePoint Online (2013 wave) deployed as a sandboxed solution and would like to notify users for approval when somebody changes an item. Having a relatively high number of fields makes it mandatory from the approver's perspective to only see the modified fields, and preferably the old values, exactly like the Version History page or the e-mail that is sent if the user subscribes for alerts on change of items in the list.

Question is how can I find the modified fields programmatically (JSON, REST) or use the native version history function?

Other interesting option would be to create alerts dynamically per item.

Thank you!