Detect which face of the cube is facing the screen

Hello everyone,
I am new to Unity. I am working on a 2D minigame that requires matching identical face of different cubes from a tower of cubes. I would like to know how could I detect the face of the cube that is facing the camera (or screen). I put on each face text/texture.
Sorry for my dull language. Thank you in advanced for your help.

Use the rotation of the cube. If the rotation around y is 0, then face 0 is visible. If Rotation around y is 90 degrees, then face 1, etc.