I have received no answer from anybody... Yes, I agree that it must not be an easy question... But I found my own answer...
As the oil is so dark that is really difficult to see it through the round little window the trick is to lay the bike on the left leg, yes, not being fully vertical. In this position there is no oil visible through the window.
Then somebody helping you moves slowly the bike to get a vertical position. During this movement you can see through the window how the level of the oil appears and when the bike is fully vertical then you can check the right level of the oil.
It works....