If there's no dipstick, then what you have to do is drain it all out, measure out the amount called for my the manufacturer, and then add it. The amount required may be stamped on the engine by the filler cap (it might say something like 900 ml), otherwise you will need to look up the recommended amount yourself in the owners manual, a service manual, somewhere online like a ktm forum, or even maybe calling a dealer and asking. It seems like a dumb setup that you can't check the oil once it's in there, but that's how most 2 stroke transmissions are.