List of finished missions/quests?

Is there a way to find out what missions/quests you have completed?

Yes and no... Story Missions after you have fin. them will have a sword(s) through them. But as for Side Quests go nope other then the ! above the quest(s)-givers head will no longer be present after completing.

I was afraid of that, but I'll just go on a complete list page and go from there. Thanks!!

As Uzar9 said, storyline missions you can tell as the icon for them will show if you have done them or not (sword/spear through the icon). Side-quest's unfortunately have no way to tell if you've done them or not, apart from the ! being above the quest giver or not.

I'd recommended opening up either the official or unofficial wiki, going through the quest list's - this way you can quickly find which npc gives a quest, go to that npc and see if a quest is offered - if no quest, you likely have done the quest already.