It depends what you want to do. If you want to check the objects height then you need to check the y axis value
if (transform.position.y > 8f)
{
Destroy(this.gameObject);
}
The other thing you might be wanting to check is the magnitude but that's unlikely, it's basically the length of the vector regardless of direction.
if (transform.position.magnitude > 8f)
{
Destroy(this.gameObject);
}