I would compare its physical size with that of a known transformer. Looking at a catalogue :- **broken link removed** ,you can see that their volume increases as their power rating increases.
If you are talking about the running current of a transformer in a circuit, put a small resistor in series with one of its windings and measure the voltage drop across it. the current in the other windings can be determined by the voltage ratio. As an example :- 230 -> 12 V transformer. .1 ohm resistor in secondary drops .08V, so current in secondary equals .08/.1 = .8A. current in primary = .8 X 12/230 = .041 A
Frank