If a post was edited, there will be an "edited [details]" box displayed to the left of the "main asker" box. I've edited this question to demonstrate. If you click on the top of that box, you will see the edit history, and that edit history is helpfully annotated so it will be easy to see what changed between versions. If no box is visible, the question has not been edited since its original posting.
Now it's worth noting that this isn't quite a complete history: roughly, if I make two edits within five minutes of each other only the second will be displayed. Similarly, if I make an edit within five minutes of initially posting the question, no "edited [details]" box will be created. I do not believe moderators can see the more detailed edit history, but I could be wrong.