Well, against Garny Gallade or Stage 2 decks, I won't feel comfortable laying down until I know that they don't run Dusknoir. The realistic thing if I see a Garny Gallade, with 1-0-1 Empoleon, with two other techs, I might believe that they don't have the room to fit in a Dusknoir. Against a Stage 1 deck, it would be hard to imagine Dusknoir in it.
Dusknoir "check" is always the first thing a player will do.
Dusknoir in Top 32, you can assume any Dusknoir in your deck is public knowledge. Dusknoir can surprize during Swiss, but never during Tops, too much scouting.