How can I detect if a user is looking at a Gameobject inside a scene from a VR headset

I've been searching for a way to detect if a user is looking at a certain object inside the scene from a VR headset (HTC Vive to be precise). Like for example: assume that I'm giving a virtual speech with the headset on and the gameobjects are 3D characters that are seated in a venue, which is the scene. How can I know if the user is looking at the characters or not and then later use this data to give the user some rating which shows how long/frequently they have been looking at the audience.