Nostalrius Begins - Quality wow vanilla realm (1.12)

any addon that can show me all the quests i've complete?

Yeah, there is an AddOn called QuestHistory, but the caveat is that it only logs the quests you have completed while you were running the AddOn. As far as I know, I don't think there is a way to query the server for your completed quests. Here is the link for the AddOn: http://addons.us.to/addon/questhistory

The API to see if a quest has been completed does not exist in vanilla. Only option is to track quests while you complete them.