If you can measure the dimensions of your tank you can use maths (a calculator may help) to work out how much oil you have now and compare that to how much you supposedly had delivered. What you don't say and probably can not prove is how much was in there just before the delivery.
Easier to use metric measurement - 1 cubic metre (meter) is 1000 litres
The U.S. liquid gallon is 231 cubic inches or 3.785 litres
The Imperial (UK) gallon is 4.55 litres.
Work out how much you have in the tank now and compare it with the paperwork. If you need to call the supplier best to describe it as a mistake rather than a rip-off - they'll admit a mistake much more easily.
Good luck... Go figure...
Peccavi
June 2009