How can I check when the centre of camera is looking at a gameobject?

how to touch off a event when the centre of camrea look at a gameobject?

I think that best way to do taht is to use raycasting.

Raycast script reference:

Use ScreenToWorldPoint function to get the center of the camera. Your argument for ScreenToWorldPoint should be Vector3(Screen.width/2, Screen.heigh/2, camera.nearClipPlane).

ScreenToWorldPoint script reference:

Example: (Attach it to an object that has a camera component)

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour 
    void Update() 
        Vector3 CameraCenter = camera.ScreenToWorldPoint(new Vector3(Screen.width/2, Screen.heigh/2, camera.nearClipPlane));
        if (Physics.Raycast(CameraCente,  transform.forward, 100))
            Debug.Log("Ou yeah!");



I didn't test this code ;)

As an alternative to the (Admittedly better) method presented above, you could just use the mouse.
You would use Cursor.lockState = CursorLockMode.Locked to lock the cursor at the center of the screen and Cursor.visible = false to hide it. Then use any of the OnMouse… commands in MonoBehavior to make the object interactive. I guess this method would only be useful in certain circumstances (like the player picking up and carrying rigidbodies), but still worth a mention.

Cursor Info,
MonoBehavior Info.