Get the property that has been modified in the editing with the help of mvc 5 .net and the entity framework

Estimads, I am a new media in this and I find the following problem, I only need to send an email if I have changed a property of an entity in the (post) edition. Otherwise, I should not do anything.

To do this, I access from the controller (editing station) of the entity in question, I retrieve the object before being edited and I compare the property that I'm interested in the new object. I do it before the EntityState.Modified. The fact is that when you get to the modification, he tells me that it's the same object and clicks.

How can I do it? I do not know if he explains me well.
Thank you very much for reading!