Untested example of checking what's changed between revisions of a IContent save event handler in Umbraco.

MyContentSmellerEventHandler.cs

using System.Linq;
using Umbraco.Core;
using Umbraco.Core.Models;
using Umbraco.Core.Services;

namespace My.App.Events
{
    public class MyContentSmellerEventHandler : ApplicationEventHandler
    {
        protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
            ContentService.Saving += ContentService_Saving;
        }

        private static void ContentService_Saving(IContentService sender, Umbraco.Core.Events.SaveEventArgs<Umbraco.Core.Models.IContent> e)
        {
            foreach (var savedEntity in e.SavedEntities)
            {
                var modifiedProperties = savedEntity.GetDirtyUserProperties();

                if (modifiedProperties.Any())
                {
                    var lastVersion = sender.GetVersions(savedEntity.Id).OrderByDescending(x => x.UpdateDate).FirstOrDefault();
                    
                    if (lastVersion != null) {
                        //Zhu li, do the thing!
                    }
                }
            }
        }
    }
}