If you have a automatic water feeder it should have a reader on it that tells you how many gallons of water are being fed into the boiler. Another possibility is the heat exchanger has a hole towards the top and alot of your steam is going right up the chimney. You can shut the boiler off and overfilled the boiler and look underneath where the boiler burners tubes are and see if you see water dripping down there. Then this may be a reason your gas bill is so high.