transformer testing
There are a couple of issues here. First, if it has multiple secondary windings, you have to use the circular mils calc and secondary voltage measurements to figure out the current of the winding you're interested in. You'll likely overload that winding before you overload the primary or core.
If it has only one winding then the entire transformer (primary, secondary and core) will likely contribute to the overload condition. When that happens, the efficiency of the transformer drops dramatically and it will overheat if run that way for very long (the secondary voltage is also well out of regulation by that point). The best way to figure out how heavily you can load the entire transformer is to monitor the input versus output power and plot the efficiency (and power loss) as you load it. But that's kind of a pain to do.
In the Amplifier thread someone posted a VA per KG estimate which is a decent way to get a ball park idea to see if further testing is warranted. The secondary DC resistance SY pointed out is another rough guide. Yet another somewhat more accurate test is to load it until the secondary voltage drops to 90% of the no-load value and use that current value.
You can BRIEFLY overload wirewound ceramic (or metal encased) power resistors by a factor of 2X - 5X if you keep the time to a few seconds (just long enough to make a reading). Although you might want to wear safety glasses  So you can use relatively inexpensive 50 watt resistors, for example, for your test loads. You can also try various heating appliances you might have around the house (clothes irons, etc.).
Audio amps are somewhat forgiving transformer loads unless you plan on doing sustained sine wave testing or driving really low impedance loads. The nature of music is such the average energy is relatively low compared to the peak power needed. So a final technique might be to just try it and see how warm it gets after working the amp hard for a few hours (monitoring it along the way).
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