I can look under my utility panel and see the tank. When I need it completely full, I watch as I fill the tank.
***Personally, I would never fill the tank until it runs out the overflow for two reasons. If the overflow were to get clogged (unlikely, but could occur) you could expand the tank and do damage. The second reason is that you can fill water faster into the tank than it can flow out of the overflow, again over expanding the tank. On most of the coaches, the grey and black tanks are sitting atop the water tank. It's just too easy to be filling the tank and walk away for a moment expecting the overflow to work and then do damage.
Don & Mary
2019 Newmar Dutch Star 4018 (Freightliner)
2019 Ford Raptor