How to make health bar disappear if you don't see enemy?

I made an enemy health bar script that makes the enemy health bar show up over the enemy's head. I don't know how could I make it so that when I don't see the enemy (have him in my line of sight), the health bar disappears, and then again when I see the enemy the health bar appears again?
Here's the script. Please help!

using UnityEngine;
using System.Collections;

public class EnemyHealth : MonoBehaviour {
public int maxHealth = 100;
public int curHealth = 100;
public float healthBarLength;

// Use this for initialization
void Start () {
healthBarLength = Screen.width / 2;
}

// Update is called once per frame
void Update () {
AdjustCurrentHealth (0);
}

void OnGUI(){
Vector3 screenPosition =
Camera.current.WorldToScreenPoint (transform.position); // Gets screen position
screenPosition.y = Screen.height - (screenPosition.y + 1); // Inverts Y
Rect rect = new Rect (screenPosition.x - 50, screenPosition.y - 12, 100, 24); // Makes a rect centered at the player
GUI.Box (rect, curHealth + "/" + maxHealth);
// GUI.Box (new Rect (10, 40, healthBarLength, 20), curHealth + "/" + maxHealth);
}

public void AdjustCurrentHealth(int adj) {
curHealth += adj;

if (curHealth < 0)
curHealth = 0;

if (curHealth > maxHealth)
curHealth = maxHealth;

healthBarLength = (Screen.width / 2) * (curHealth / (float)maxHealth);
}
}

 

I guess, you should use Raycast from your player to the enemy. And if it is hitting the enemy and nothing between those two objects (and maybe also has a certain distance), then you know it is insight and you can show the healthbar.
http://docs.unity3d.com/ScriptReference/Physics.Raycast.html

 

Just check if the render is visible to the main camera and turn that object off.

Look at the Render class and find isVisible property.

 

If he is using a first person thing or 3rd person, than this will work. If it is like a top down thing, then he has to tell us as the camera would "see" the object even if the player should not.

 

ah, then when you don't "see" the enemy as you say have him in your line of site,

a raycast pointing in your forward direction would be a good way to detect it.

 

I use third person, and i want the health bar to show up whenever my main camera sees the enemy gameobject

 

Then you can use SubZeroGamings advice, as this is what you are looking for. If the it is visible to the camera, show it.