Spying

The rules for spying begin on DMG 18.

Though the book says the success table is for NPCs, there is no reason it can't be employed for a henchman or even a PC assassin if the mission is happening during downtime. If the DM plans to use the success table in this case, he should make sure the player is aware of that intention so the player is not surprised by the abstracting of the mission to dice rolling.