Unless the casting of the case is porous, (Unlikely.) there are only a few places it can leak from.
The filler, level or drain plugs, the joint between pinion carrier and swingarm or pinion carrier and box casting, the crow wheel plover plate gaskets or one of the two crown wheel carrier seals.
The plugs and pinion carrier leak points are very easy to check visually. The seals require removal of the wheel.
If the oil is dribbling down the face of the box over the crownwheel cover plate but the inside of the crownwheel carrier, (The splined bit that the wheel fits into.) is dry of oil it will be the large, visible seal in the crownwheel cover plate. If though oil is running down the inside of the crownwheel carrier it will be the inboard, smaller seal which requires disassembly of the box to get to.
Pete