I wish I had the money for a fancy water level…
I was having some issues getting my dartboard leveled, so I came up with this dart arrow. The flight is just sized as the outer single area. Offer it up and place it as accurate as possible over the area. Once happy with the location put one of your darts in the hole in the flight. The arrow should point downwards. For extra accuracy you could put the barrel of another dart in the arrow.
The tip should point straight at the bullseye. If it doesn't, your board isn't level. Turn until the tip and bullseye line up.
The author hasn't provided the model origin yet.