Enter vision mode by toggling the 'eye' icon (next to the camera controls at top-center of the screen), or by pressing and holding the shortcut key ('B' by default).

In vision mode, the on screen contextual action menus are replaced with the following items:

• Guards/Cameras/Etc: Vision preview. While hovering this icon, only this guard's vision is visible. Click to toggle hiding this guard's vision from the UI (example: if you're ignoring camera vision and just trying to step around the guards with guns).
  
• Pulse Drones: Effect preview. While hovering this icon, tiles reached by the pulse scan are highlighted.
  
• Grenades: Effect preview. While hovering this icon, tiles reached by the explosion are highlighted.