You can use Raycasting to do this.

Cast a ray from the enemy object to the player object, and check the hit results. If the first hit result is the player, then the enemy can see the player.

For example, in your Enemy script (make sure the "player" variable has a reference to the player in it):

var player : Transform;

function Update  () {

    var hit : RaycastHit;
    var rayDirection = player.position - transform.position;

    if (Physics.Raycast (transform.position, rayDirection, hit)) {

        if (hit.transform == player) {
            // enemy can see the player!
        } else {
            // there is something obstructing the view
        }

    }

}