>>213106819he didn't really know, merely had a gut feeling about there being something off with dexter.
in the books it's explained away by both dexter and him, as well as many other fucked up individuals being possesed by moloch or some shit, which allowed them to recognize each other's nature to a certain degree.
iirc it's even mentioned in the show but is explained away as being some sort of hidden sense.
they turned those supernatural elements way down in the show and instead it became a perma-angry darkie hassling a seemingly autistic co-worker.
>>213107361they portayed him as an obsessed and neurotic spazz that didn't really know shit but just had to uncover dexter's secret