[@Light Lord]No He could tell that Devon was a human because of the Echo. Devon isn't the first Human turned pokémon that Zeth's have encountered. He din't really act upon it and do not intend to either for several key reasons. 

He also felt Pain from Celty, he's not sure why yet. But he happened to pick up her name during it. 

And yes that could be the case if he wanted to read someones mind. 

Which is a 'If' that he might not even use unless there's really a reason. 

Like if he encounter a pokémon he believe is telling untruths and gibberish. An obvious bad pokemon per se