Depends on your DC panel. The holding tank and "Panel 16" have no sensor or indicators. I've never seen one that actually works anyways and cleaning the sender is no fun at all.
Oh, depends on what brand your portlights/hatches are. My hatches are Gebo, and spares can be found online, although I got lucky and picked up a bag for next to nothing from a chandlery "used" shelf. They can be disassembled and replaced easily.
My portlights are Rutgerson and I figure spares are probably available from Sweden, but haven't needed any yet.