that'd be a complex & loooong story... to cut it short, he used to contraddict himself on pretty much everything, regardless of the 'personality' he was using, had the very same 'style' of writing (same mistakes in composing a sentence etc), never gave some basic details when asked... had an extremely weird attitude (to say the least) about lots of things, used to write about things/places/people he/"they" had no idea about etc etc etc.
but that's just a tiny part of it all.
when he's been finally 'discovered', he confessed it himself.
i'll leave the further explanations/details to Cerulean...