None that is official, you could just search the workshop though.

Why should there be such a mod?
I understand your concern, but in my opinion it doesn't make sense...
Where is the learning effect?

you can also set the keybind for tag in game to be left click so that whenever you shoot/swing your melee at them it will tag. In my experience this makes it feel as if it does it automatically. If you're searching for something that makes it easy to tell when they are approaching, you should up the enemy audio in game and learn the sound effects for each special. After you get used to it, it will become pretty distinct.