This is a difficult question because data centers typically pass along electric usage costs to clients to avoid being treated as a utility company (whole new set of rules). Thus, if they meter the power they factor in cooling and power costs plus expected power loss at the cost the utility charges them. If the power is unmetered, the best rule of thumb is look on your servers in each rack add up the expected power usage and divide by two (servers generally run at 50% power utilization or less). Find your actual cost and see what they are billing you.
You can also get a power meter. These are little hand held units which have a large claw which goes around a cable. They can tell fairly well how much power is going through the cable without having to disconnect anything. Simply measure each main power cable doing to the rack and add all the numbers up. The meters can be bought at most hardware stores and cost a couple of hundred dollars (US).