Perhaps a third way (still in the rough) is to guesstimate the transformer capacity. This starts from the assumption that for suchlike transformers, taking copper loss as about 4% is normal.
One measures the voltage on a secondary winding and loads another until the first voltage has dropped by 4%. The loaded winding will necessarily be momentarily overloaded, but the resultant primary current*primary voltage will be a fair indication of total capacity.
Or one can simply measure the primary d.c. resistance and calculate what current through there will yield a 4% voltage drop, which will be close to the maximum working primary current. This can be less accurate because primary copper resistance is often a few ohm only, and most DVMs do not measure resistance accurately to the first decimal. If there are several (low voltage) secondary windings, division of energy can certainly be a vague matter.
