You need to use Raycasting to detect that. Cast a 2D ray down from the input location and check if it hits something. Here's good info about it.
if (Input.GetMouseButtonDown(0))
{
var hit : RaycastHit2D = Physics2D.Raycast(cam.ScreenToWorldPoint(Input.mousePosition), Vector2.zero);
if(hit != null)
{
Debug.Log("object clicked: "+hit.collider.tag);
}
}