title: How to detect if form field has changed without using hidden field
body: <p>I am using ASP.NET MVC 3 and have a need to detect if the form field has been changed on the server side. I know about using tricks with hidden fields, but I was wondering if there is a way to do it by just using the API?</p>
  
  <p>Basically, I have edit screen for my model and one of the fields is an optional id that can be specified. If the field is specified, I have to insure it is unique (no other model has it). So on the edit controller, I want to run the validation but only if that field has been changed.</p>
  
  <p>Please note, I don't need to know previous value vs. new value, just if the field value has changed.</p>