If the oil is getting past the valve stem seals and being burnt you will see a fair bit of blue smoke out of the exhaust when you start the car, but it is unusual to lose a lot of oil by this route. A black coating inside the end of exhaust pipe will show a rich mixture of petrol to air and this sometimes happens on cars only used for short runs, burning oil does not normally result in a black coating inside the exhaust pipe but may leave an oily film. The main reason engines use a lot of oil (apart from leaks) is when the bore or rings are badly worn and oil is left on the cylinder bore, again you will see this as blue exhaust smoke. When you rev the engine black smoke is petrol, blue smoke is oil.