Heating oil tank (plastic tank) - how to read the fill level?

Hey guys,
in my purchased home, the heating runs on oil. In the oil cellar, there are 3 × 2000 liter plastic tanks for this purpose. Unfortunately, I can’t find a direct indicator of how much oil is still inside. Each tank has 3 openings each and all are already "occupied", so I cannot install a tank gauge or similar. However, on the last tank, there is a kind of "measuring stick" (Picture 1).

At least, that's what it looks like. A cable runs from it to the oil connection outside, where there is a 2-pole socket (Picture 2).

Anyone has an idea if – and how – one can read the fill level from it?

This is not a dipstick, that part simply switches off the pump in the tank truck when it delivers oil.

Plastic tanks are somewhat translucent, oil-colored, you should be able to see how empty they are. Karsten

Our 1500 liter tanks are about 150 cm high, so 1 cm equals approximately 10 liters, and since, as already mentioned, they are slightly transparent, especially when you hold a lamp against them, you can easily determine how much quantity is still available.

Hm, then the tanks are probably empty, because I can't see anything at all. I had specially gotten a tank gauge with ultrasonic measurement and a wireless display, but then noticed that there is no connection on the tank to connect it. Too bad... then I'll have to make marks on the tank when oil comes...

What is it like if you once hold a folding ruler [ ] from above and go down to the floor with it? Then pull it back up and see where the level is. Just like with the car [ ]