How Do I Know When My Tanks are Full?

When it shows full its full! Dump asap! Dont max out the capacity it could prove to be a messy mistake. Sometimes the lights on the blackwater tank will show full when indeed it is not.....you can always check by looking down when you flush. The greywater overflows usually in the shower stall. Thats fine unless you are storing something there and forget about it until it gets wet.