How to calculate secondary winding max current output?
The resistance of transformer windings can be a useful hint to people who know what they're doing when trying to figure out how a transformer is arranged internally. Unfortunately, it tells you nothing about rated or expected currents, or the voltages that are expected to be present on the windings.

Find a data sheet for it, or a piece of equipment that uses it and measure some voltages, before you apply any voltage to this, and certainly don't simply plug it into mains and see what it does.

The best way to start investigating an unknown transformer is to get another transformer that can deliver a safe low voltage, say 12v AC, and connect that to the highest resistance winding pair on the unknown transformer. Now make some voltage measurements to establish turns ratios.

Even after this, you still won't know whether a single high voltage winding is meant for 120 or 240 volts. Two identical high voltage windings are probably 120v each, intended to be used series or parallel. Several very low voltage windings connected to a high voltage winding are probably tap-change windings.