Mains transformers

I have a fair number of mains transformers, unmarked, sitting in a box.
It's fairly easy to find the output voltage, but what is the best way to arrive at a current rating? At present I add more load until I'm not happy with the transformer's temperature, then deduct 20%. Any better ideas?

You're about right but you must wait for a sufficiently long time for the transformer to stabilise in temperature.
I would rather do it this way.

1. Add load until output winding voltage drop 6-8%.
2. Connect this loading for many hours and then feel the transformer temperature.
3. Add or drop a bit of load.