// Finds out whether target is on the left or right side of the screen
var target : Transform;

function Update () {
    var viewPos : Vector3 = camera.WorldToViewportPoint (target.position);
    // viewport coordinates range from zero to one
    if( viewPos.x > 0.5 )
        print ("target is on the right side!");
    else
        print ("target is on the left side!");
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Transform target;
    void Update() {
        Vector3 viewPos = camera.WorldToViewportPoint(target.position);
        if (viewPos.x > 0.5F)
            print("target is on the right side!");
        else
            print("target is on the left side!");
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public target as Transform

    def Update():
        viewPos as Vector3 = camera.WorldToViewportPoint(target.position)
        if viewPos.x > 0.5F:
            print('target is on the right side!')
        else:
            print('target is on the left side!')