How is the ability of a Devil Fruit discovered? Since there is just one Devil Fruit for a specific ability, how does one know about what ability it holds?
For example, Buggy and Teach knew about the powers their Devil Fruits would have provided them.
It is mentioned that they had read about it.
But if the fruit occurs only once till the user dies, how did they find out about the fruits' powers?
There are a number of ways to know the abilities of a Devil's Fruit.


There's a Devil's Fruit book in the Grand Line, which lists several Devil's Fruits. Sanji mentions this book in Chapter 464. Spandam and Teach also make a mention of it.


For Devil's Fruits whose ability is not known before, it can only be found out after actually eating it. In Chapter 385, when Kalifa and Kaku eat unknown Devil's Fruits, Lucci tells them that they will begin to sense the changes in due time.


When a Devil's Fruit user dies, their Devil's Fruit will respawn, and hence its abilities would be known. 


In SBS Volume 48, the author Oda Eiichiro promises that a certain professor will eventually appear in the story, and explain what Devil's Fruits really are. We can expect to know more about identification of Devil's Fruits when that happens.