Unlocked Skins
There’ll be something in the info pop-up that’ll tell you. It’ll either say locked or unlocked.
Its on the tooltip that shows up when you hover over the item. Its underneath the stats / runes / sigils on the item but above the rarity. It’ll be white and say “Skin Unlocked” if you have the skin, or be greyed out and say “Skin Locked” if you don’t.