Check if anything is rendering to a render texture

Hi everybody,
I have several cameras, all of them rendering to the same render texture. These cameras belong to homing missiles fired by the player. This works well : the last spawned camera being the one rendered.

I need to check if any camera currently renders to the texture ; if not (which means that all missiles have been destroyed), I want to disable the render texture.
How can I check what's being rendered (or not rendered) to my texture ? Thanx for help.