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I am not sure what you mean. One indicator is the mutual conductance, gm, this is the change of current through it per volt change on its gate. So hook up a common source circuit, with some fixed bias. Measure the drain current, change the bias and see what the new current is. Do the maths.
The current gain is infinite as there will be no input current to the gate, but there is an output current.
The voltage gain is gm X drain load resistor. As the drain resistor gets bigger, the change of gain decreases as the drain has a finite resistance internally. So if you plot the gain v. load resistance you will see what the ultimate voltage gain will be with an infinite load resistance.