This achievement can be earned in the Manifest destiny on the Vice Desk.
Once you arrive on the scene of the bus shooting and you gain control of Cole, go left to the end of the building and take the first right up the ally. Continue past the parked truck and around the next corner to the fire escape ladder. Climb this to the roof, your appear behind a stairway exit, take cover on this corner and gun the sniper down.
Once this is done, check his sniper rifle (not needed for this achievement but thought I'd mention it) Make sure to check his right breast pocket to find Snipers Pocketbook. After this your appear back down near the bus where Cole will recognize the driver of the bus.
Talk to the driver and ask the questions...
Informed Of Coolridge Heist - Bad Cop
Motive for Shooting - Accuse and use the Snipers Pocketbook.
After his done speaking your unlock this achievement.