Power supply amperage
The case for home appliances usually has the power consumption written on it.   That will give you an upper bound if you assume say 80% efficiency.

You could hook a fat power resistor up chosen to dissipate say 1A and see if the output voltage drops indicating overload.

Really, you could do better with a dual-rail PSU from some obsolete computer equipment (eg Zip drive), available really cheaply at flea markets etc.   If you only want 5v, old mobile phone chargers are good; prefer switchmode (light, efficient, well regulated) over transformer (heavy, inefficient, poor regulation).