How can you tell the difference between trush or just milk in a 1 week old newborn baby's tongue?

Usually : Thrush has redness under the white area and it wipes off... If you give your baby water and the whiteness is gone it is probably milk.

Maybe: Thrush is common in early infancy & sometimes hard to tell from a milk-tongue. The tongue surface at that age can trap milk solids in the surface & you may not be able to scrape/wipe it clean. Unless their is a powdered milk like layering covering the non-tongue surfaces (inside the cheeks, lips, gums) i don't consider it thrush.

Milk will wash off: Thrush can not be cleaned off easily.