You can open the toilet valve and see if there is water down there, and yes you'd get water in the bottom of the shower if you overfilled. But you won't have a full black tank before you get a full gray tank if you dump them both at the same time.
What I do is fill the fresh water tank right after I dump (there is usually a clean water hose for that purpose at the dump station). I never connect to city water. Since all three tanks are the same size that means that i can't overfill any tank before I run out of fresh water. And that way I know I'm never carrying more than 30 gallons (250 lbs) of water total between the 3 tanks so I can't end up inadvertently overloading the trailer. The little lights work pretty well on the fresh water tank because there is no contamination, but if they fail its not a big deal, you can see the level in that tank by looking at it because its translucent.