Black Tank Valve doesn't close completely

[aatauses]

I have a similar issue and have thought about adding the extra dump valve at the outlet. I have never looked at the valves from underneath, can you tell by looking if they are closed or not---I sometimes have clear water and sometimes have black water so not sure what valve may be malfunctioning---that is why I am asking if you can tell by looking at the valve weather it is closed or not?
thanks
al