Instructions
Look for a lid. Without that, the item in question is either not a tureen or, worse, it's incomplete. The lids should either have a handle or a large finial in the shape of a creature such as a lion, bird or dog.
Examine the general shape and size of the item to determine what type of tureen it might be. By far the grandest in proportions is the soup tureen. These are usually oval, with handles at either end. Sauce or gravy tureens echo the same design but are much smaller, because there would be several on the table at once. Dessert or cream tureens have a distinctive boat-shaped bowl on a raised foot. Vegetable tureens are square with rounded corners.
Check for signs of age. On soup and sauce bowls, inspect the interior for crazing (small, cobweb-like lines) caused by hot, greasy liquids. Check the finials and handles to see whether they are chipped or have been broken off at some stage. Feel around the lid for chipping, paying particular attention to the vulnerable ladle aperture. While heavy damage is unwelcome, vintage pottery tureens lived full lives and some wear and tear is to be expected. A completely pristine example should make you question its authenticity.