I use something like this for filling/topping up diffs/gearboxes:
https://www.amazon.co.uk/Multi-Use-Tran ... 077&sr=8-3
Generally (details vary slightly, from box to box) there's a drain plug somewhere on the bottom and a fill/level plug somewhere on the side. Checking the level is a bit tricky, generally unscrew the fill/level plug out of the side and see if oil runs out. If it does there's plenty/too much in. If it doesn't then poke a finger in and see if it comes out with oil on - level about right. Or just pump some more in until it runs out.
I've given up checking levels, I just tend to drain it and refill completely at proper service intervals.
If it's a Triumph "single rail" manual box.... they blow a bit out of the breather and the selector rod seals can also leak. Generally the top of mine (Spitfire, not Dolomite... same box) is damp with EP90.