You always open the fill plug first, this will ensure you have not drained the gearbox oil and now cannot open fill plug in case it does not agree with you.
It depends on what type of cars you have, some cars have dipstick on their transmission and some don’t have, However, most car don’t have a dipsticks in their transmission. If that’s the case, you need to jack up your car evenly and undone the drain plug if fluid comes out and drips it means you are good to go if no oil drips then it means your transmission oil is low and needs topping up. Hope this helps. Cheers
