In chapter 385 Lucci states > You learn the name of the fruit when you gain its
power - so why did Luffy not learn the name of the Human-Human Fruit, Model:
Nika after eating it? - or did he mean you name the fruit yourself after
learning of its abilities?
They have devil fruit catalogs but often times the power/name does not have a picture of the fruit prior to being eaten. So basically once they find out what powers they have they can match it to an existing fruit.