How detect player is which side of box?

You just check the player's coordinates and the box's coordinates:

 if(box.transform.position.z > player.transform.position) {
     // player is on side 1
 }
 else {
     // player is on side 2
 }

If this is supposed to work not matter what the box's rotation is, you'll have to transform the player's position to the box's local space, and check if the player is below the box' position. In the box's local space, the box's position is (0, 0, 0), so in that case, you just check if the player has a negative z:

 if(box.transform.InverseTransformPoint(player.transform.position).z < 0) {
     // player is on side 1
 }
 else {
     // player is on side 2
 }

If you only want to know this while the player is touching the box, put it in your OnTrigger/CollisionEnter.

Here is answer worked from Iron-Warrior

What you're really asking is to test which side of a plane (your box wall) a point (game object position) is on. A plane is defined by a normal (the direction it's pointing) and a location. To check which side of it a point is one, you take the direction from the origin of the plane to the point. If the angle between the direction and your plane's normal is greater than 90 degrees, is on the backside of the plane.

Taking the origin of your plane to be the center of your box...

 Vector3 direction = playerPosition - planeOrigin;
 if (Vector3.Angle(direction, planeNormal) > 90)
 {
 // do something
 }

This is a good way of testing this since it works in all directions and all dimensions.