There are two holes on the gearbox that are designated for filling and draining the gearbox oil, the one at the bottom is for draining and the one on the side is for filling. On a flat ground ,you fill up the gearbox until it starts to flow out of the hole on the side which your filling from, at which point you know the gearbox is filled to the correct level.
So you can remove the plug on the side and see if gearbox oil starts to leak from the side and top up as necessary. Our gearbox don't have a level dipstick.