How do I know if I already have disenchanted a trait?

In ESO, you can disenchant traits on armor and weapons for creating that same trait on new armor and weapons.

Is there a way to know if I've already disenchanted the trait on an item? I'd like to know if I can go ahead and sell the item or hang on to it to disenchant it later.

Solution 1:

If you go into the researching tab (the last tab when in a crafting station) you will see the traits and their associated gems. Ones that are blacked-out/grey are traits you do not yet have researched. If you have a trait researched you will clearly see the color and if you try to research a trait you have it won't let you anyway.

There is no way when with a merchant to check before selling the item, you'll have to individually check enchanted weapons/armor at their crafting station beforehand.

The trait wiki shows all about the traits and getting them.

Solution 2:

I found a nice add-on on ESOUI for this, called Research Assistant (http://www.esoui.com/downloads/info111-ResearchAssistant.html)

This add-on places small markers against items in your inventory to indicate various things, including traits you have not researched, and the 'lowest-value' item in the case of duplicate traits on items.

This means that you can see what (not) to sell when you speak with merchants.

Worth noting that this add-on also marks intricate items which give you more XP if you deconstruct, and ornate items which are worth more to sell.