The power can be tested with a signal generator, a SS amp, an o-scope and some resistors. Drive the secondary, with its rated load in series with the SS amp. Apply a load equal to what is reflected from the secondary. This will take some power dissipation, so proceed with caution as things can be hot, and the AC voltage can get very high. Voltage divide the primary's voltage so it does not crisp the measurement equipment. Apply signal, measure amp output voltage, and the fraction of voltage across the voltage divider transferred to the primary. Plot them.
cheers,
Douglas