There are not, too many, places it can leak from: between the mainbox and the t-box itself which would mean the input seal, out the front or back flange seals, out the speedo drive (probably wouldn't give you that much oil) or the big access plate on the bottom which is where mine wept from. It could leak from the speedo drive housing to main housing seal but doubt it there are other places it could leak from, but unlikely for starters.
I'd just give it a good clean with a rag and some WD40 or GT85 or similar and then take it for a reasonable drive then take a look under. Some of the leaks will be obvious without cleaning them.
All cheap to fix but a pest if it's the input seal as you need to pull that big unbalanced lump off.