It does say so, right hand side "Last Edited 18 minutes ago!
Now 19 minutes ago.....
Now 23 minutes ago.....
Now 38 minutes ago.....
As John explained a long while ago.
If you edit your post before anyone has added a comment, it doesn't show. But once anyone adds another comment after yours, and you edit your post, a flag shows, right hand side, Last Edited: whenever.
Or, even if you edit your post, before anyone else has commented, but more than 30 minutes after posting it (not sure of precise delay) it still adds the flag anyway.
So this post now has a flag, showing it has been edited. Aah, it looks like the delay is more like 20 minutes.
Additionally, when you're about to respond, before you submit your post a warning comes up to tell you if there are any new posts since you started your response, and before you submit it.