You must identify the item to see if it’s an Ancient.

When dropped, Ancient items share the same orange glow as regular Legendaries.Only drop for characters who have reached level 70, and only on Torment level difficulties.Any percentage-based affix will not be boosted (such as Attack Speed, bonus % to skills or % Critical Hit Chance/Damage).You can’t equip more than one item with the same name, even if one is Ancient and the other isn’t.Set items are also able to roll as both Ancient and Primal Ancient.The stats on these items will (generally) be tailored to the character class you are playing when the item drops.They are identical to Legendary items except that their base attributes (damage or armor) and all non-percentile affixes roll higher values for both minimum and maximum possible rolls. In general, Ancient and Primal Ancient items are more powerful versions of Legendary items. For those of you who may have taken a break from gathering loot recently, we thought a quick refresher was in order to clear up any confusion about the differences between Legendary, Ancient, and Primal Ancient items.

Ancient items truly shine in Season 17: so long as you have no Set bonuses equipped, each Ancient you equip will provide you with a massive 750% boost to damage dealt and 4% reduction in damage taken.

Diablo 3 reforge legendary to ancient