You can check the oil level by removing the filler plug and putting a home made "dipstick" in to check how low below the filler plug the level is...
Home made "dip stick" - I just use a cable tie bent at 90 degree's works wonders
I check the oil in the box once every few weeks... its low at the moment, but then I can't be arsed to fill it up as a new (PHATE?) box will be being fitted soon with LSD and new seals!