Where to find the specifications

Listed on the furnace

Somewhere inside the service panel of the furnace, there should be a schematic for the furnace. This may list the electrical specifications of the blower motor. If not, the blower motor itself will have a label on it. It should list the voltage, amperage, horsepower, etc.

Using an Ammeter

An ammeter is used to measure the amount of current flowing through an electrical circuit. You can use a clamp on ammeter to measure the amount of current being used by the blower motor while it's running.

Simply set the meter to measure current (A in the pictured meter), and clamp it on the hot wire feeding the motor.
_{This method may require working on or near live wires, please use caution and/or consult an expert to take the readings}

Calculate Usage

With the above information and Ohm's law, it's easy to determine how much power the motor will use over a given time period.

Ohm's law says that Power (P) = Voltage (V) x Current (I), so we can determine how much power the motor uses at a single instant in time.

ex.

P = 115V * 5.2A
P = 598 Watts  

Next we have to determine how many kilowatt hours the motor will use, which we do by dividing by 1000.

P = 598 W / 1000
P = 0.598 kWh

So for every hour the motor runs, it will use 0.943 kilowatts of power.

Total power/day = 0.598kWh * 24h
Total power/day = 14.352kWh
Total power/month = 14.352Kwh * 30
Total power/month = 430.56kWh

Finally, if you multiply this number by the amount you pay per kilowatt hour. You'll see just about how much it costs to run the motor constantly.

Cost = 430.56kWh * $0.07
Cost = $30.14 per month

_{Notes:
Results using this process will be an estimate only.
Values used are not actual values, they are example values only.
Calculations may be incorrect, nobody checked my math.}

Why your various meters don't do a good job measuring power being used by various devices is due to an AC Circuit phenomenon known as Power Factor.

Power Factor The ratio of real power to apparent power.

Real power is the work done, apparent power is the product of the current and voltage in the AC circuit.

Capacitive and inductive loads cause a phase change between the current and voltage.

Inductive loads cause current to lag voltage and capacitive loads cause current to lead voltage. Think of two sine waves, one representing current and the other representing voltage that get offset by up to 90 degrees depending on the type of load. This phase shift messes up the reading of the real power consumed by the circuit. And if the power meter does not measure power factor and correct for it, you basically have a good door stop.

If you have a lot of motors in a plant, you often have to add capacitors on the power line coming into the building so you don't mess the power grid up. It is done to reduce the phase difference between current and voltage.

So, if you're going to measure AC Power, the device must measure True Power (Watts), Apparent Power (VA), Power Factor (%PF) in addition to Voltage (RMS), Current (Amps) and Frequency (Hz). Once you have the True Power and Time, you can read kWH consumed.