Electricity / kwh usage through the roof for no apparent reason. How do I find out what's causing it?

A lot of times these hydro meters are all located in a metering closet somewhere near the main service. Your meter could be labelled incorrectly, so you are paying for someone elses hydro and they are paying for yours. I would start there. Have a contractor trace it out and give you a definitive answer... Somebody could be stealing your power but the contractor could tell you that as well.