Is there a mod that adds dialogue to the Master Trainers that makes it to where they actually tell you that they train a skill to 100? I'm trying to play through Morrowind without using hints and whatnot on where things are and stuff.

from my understanding it's always the skill at the top of the list... but i could be wrong

http://www.lgnpc.org/mod-secret-masters

To answer the question, no. I don't think there is a mod that lets the NPCs identify themselves as Masters. (I don't think the LGNPC mod has the Trainers necessarily identify themselves as Masters)

But you can find the info on the http://www.uesp.net/wiki/Morrowind:Master_Trainers
*Note that you will need the Patch Project to actually get training from some of them.*

The LGNPC mod is definitely recommended though. Adds more details to the Trainers.

This doesn't make master trainers go 'Why hello there, I am a master trainer.', like requested. However I suggest using it. In fact, I suggest using every LGNPC as they include some very nice stories and quests.

http://arcimaestroantares.webs.com/antareslittlemods.htm