In fact, some loaders have this device. A section of steel bar or angle iron of several tens centimeters can be welded above the pin shaft at the front end of the intermediate oil cylinder (hopper oil cylinder), and the other end of the steel bar points to the rear end of the oil cylinder (the oil cylinder is parallel to the steel bar).
After leveling the hopper, weld a mark (visible in the cab) at the corresponding position between the rear end of the steel bar and the oil cylinder, so that when you see that the rear end of the steel bar reaches the mark, the hopper will be in a horizontal position.
There are many loaders in this equipment, you can refer to it.