Not sure about checking the level, but to fill it up, theres a cap that screws into the side of the gearbox, or a sump nut (square drive, not sure on the size), this will be at the front of the engine, facing towards the radiator (well on the mk1 clios anyway)
Basically to fill it up just use a bit of pipe or if u can get the little squeezy bottles of gearbox oil, they seem to do a good job.
When oil starts coming out of the filler hole, its full lol.
Make sure you do it on level ground