how do you check the oil in the differential? i dont know how much i have in it cause its leaking but i dont know if its leaking bad or if its just slowly been leaking out over the years. also, what kind problems would i experience if there wanst enough fluid in there? and finally, what type of oil would i put in there?

whats the car and motor?

that would probly help alot.

Usually there is a plug somewhere on differential that you remove to put fluid in. If you remove the plug and can stick your finger in there and touch fluid then there is enough in there. If you can't touch fluid put the correct type fluid in that hole until it runs out then it is full.

thier is a couple different plugs used here.
One is just a black rubber plug,
and the other would screw in, use a 3/8 ratchet drive to unscrew this type of plug.