If you've leveled all your DoH classes and obtained all the available Master books, a search by item name in your crafting log will let you know if you can craft it. Being able to do the search from inventory would be nice QOL but you still have the other option.

Generally, marketboard value is the way to determine if an item is worth holding if you don't have an immediate use. If the item won't appear in a marketboard search, then you know it's not crafted (collectables and other DoH use specific items not intended for trading the obvious exceptions). If you received it as loot in a dungeon/trial/raid, it can't be crafted (though loot drop gear can share models with crafted items if you're thinking about glamour).