GeometryUtility.TestPlanesAABB

Description

Returns true if bounds are inside the plane array.

Will return true if the bounding box is inside the planes or intersects any of the planes.

The TestPlanesAABB function uses the Plane array to test whether a bounding box is in the frustum or not.
You can use this function with CalculateFrustrumPlanes to test whether a camera's view contains an object regardless of whether it is rendered or not.
The test is conservative and can give false positive results. A bounding box can intersect the planes outside of the frustum because the planes are infinite and extend beyond the frustum volume. A typical false positive result is produced by a big bounding box near the frustum edge or corner.

Additional resources: GeometryUtility.CalculateFrustumPlanes.

using UnityEngine;

public class Example : MonoBehaviour
{
    // Detects manually if obj is being seen by the main camera

    GameObject obj;
    Collider objCollider;

    Camera cam;
    Plane[] planes;

    void Start()
    {
        cam = Camera.main;
        planes = GeometryUtility.CalculateFrustumPlanes(cam);
        objCollider =  GetComponent<Collider>();
    }

    void Update()
    {
        if (GeometryUtility.TestPlanesAABB(planes, objCollider.bounds))
        {
            Debug.Log(obj.name + " has been detected!");
        }
        else
        {
            Debug.Log("Nothing has been detected");
        }
    }
}