When trading with librarians, you'll see the enchantments in their books. However, you shouldn't use large or auto GUI, or there just won't be enough space to show them.
This shows the trades list left of the GUI, with JEI to the right.
This image shows trading with a farmer, on a large GUI scale. The farmer locks the wheat and pumpkin pie trades. All other trades are allowed, but the player doesn't have potatoes, carrots, or pumpkins so these trades won't work.
