How do we check the maximum load of transformer and generator?

U need to to know the transformer nameplate. Short the primary terminals & apply a voltage in the secondary (rated voltage multilied by the %impedance). Check the primary and secondary currents which should be equal to rated full load currents. Measure the oil & winding temperature which should not go beyond declared maximum temperature. A time will come when the temperature becomes stable. Then your test is complete. Do all the tests at principal tapping.

by calculating the connected loads

then KVA=3*I*V