I don't know how to have it checked by the gas utility provider, but I can tell you something about conversion factors.
Gas meters measure a volume of gas (in cubic feet - the newer meters may measure min cubic metres).
However gas is billed to you as a price per kWh (kilowatt/hour) - the same type of measurement system used for pricing electricity.
With gas there is a conversion factor that says that a certain volume of gas when burned will produce a certain amount of heat (kWh). The approximate conversion factor is 100 cubic feet is around 30.5kWh.
Trouble is, not all gas has the same calorific value - it depends where it came from. Utilities have to work out what value the gas in your area has. I've no idea how they do this - but an error in this conversion factor results in wrong billing.
Good luck.
PS I suspect it is a storm in a tea-cup - conversion factors aren't going to vary by much - at a guess +/- 3%. So don't expect a big refund - equally you may find you have been paying too little.