Through his sleek salesman talk I could discern duplicity.
Through the trees I saw a house.
I saw through the trees.
I saw a house through the trees.
The idea here is the speaker can, using a visual metaphor, 'see through' the sleek talk to something the talk doesn't quite 'obscure' - duplicity.
The word through has numerous meanings as a preposition, adverb, and adjective. As indicated in several comments to the question, its exact meaning in the above sentence is not obvious.
It could mean by means of.
Based on his sleek salesman talk I could discern duplicity.
It could mean getting past the limitations of
Despite his sleek salesman talk I could discern duplicity.
The latter interpretation is comparable to the use of the phrasal verb see through
Through his sleek salesman talk I could see duplicity
While discern through is not a standard phrase, it has a similar sense.
The original sentence makes clear that the speaker has perceived the truth, but it is not clear whether the salesman's talk was an overcome obstacle or a facilitating clue.