There will be a dotted line if the post was edited. You can hover over it to see the date thereof.
Note: a post will not be shown to be edited if it was edited within 10(?) minutes of posting
Where is this dotted line? I just edited a very old post of my own and don't see it. Could it be forum specific?
Below is an example. It is under the post creation date. If you hover over the date, you should see the date of the last edit.