devil fruit user

I wonder, how do people know which fruit to eat? I mean, is it possible to know what exact abilities are provided by a perticular fruit? Or you learn this only after you've eaten it?

What's everyone's thoughts on this guy? Is he a mink that simply can't use electro? Not out of the question, since we've never seen Bepo use it and he was separated from Zou since childhood IIRC. Electro could just be a cultural thing for the minks. OR, is he a user of a variant Hito Hito no Mi...