To know the maximum transformer load power, one must combine wire and core loss. A practical method is to connect the transformer to a nominal mains like 220 V AC, and use a high-power resistor combination as a load on the secondary.
One rule says the output voltage may drop by 10 % under a maximum load but the combined core/wire loss may cause overheating after some time.
I used thermistors to monitor core AND wire temperature and let the trasformer with that 10% down load run for at least five hours. If the temperature rises not higher than 60 degrees, the transformer I deem safe. If it tends to rise higher, I can use a small fan to reduce the temperature. But try to keep it all safe, under 60 degrees.
Some modern transformers use green-enamel wire that allows to solder the ends without removing the enamel. Those wires may not be safe at 60 deg.C