If your differential is not making any noise, then replace the fluid at the scheduled mileage interval recommended in the owner's manual. That's all the maintenance required.
look on the rear it self and see if there is a square nut on it, might be sticking out or recessed, that is the fill hole and the only way to check the oil level, once removed you can add the proper oil in till it flows out
look on the differential. There is a fill/inspection port located on the differential, usually on the rear facing side. Remove the bolt and look inside. The fluid should be level with the hole.
Since the differential is sealed with no dip stick there is not way to perform a maintenance type check. Consult your owners manual for the fluid change mileage.
