[A/S] Holding Tank Question

[asdf1009@xxxxxxx]

Okay, folks -- 

Here's another possibly silly question from a newbie: How do I tell if my 
black water  tank is full?  I have no gray water tank, there are no guages of 
any kind - and I don't want to just wait for it to back up, especially when 
gray water runs in there, too.
Do I just guestimate how long it will take?

As always, thanks - 
John