1. Set the DC gain to 1 by connecting op. ampl. output to the (-) input via resistor (the resistor value is not critical , say 10 - 30 kohm).
2. Set the low frequency AC gain to the open loop gain by shorting the negative input to the ground via capacitor. Its value is not critical, but the lower resistor the bigger capacitor is needed, for the start the time constant should be greater than say 10 sec.
3. Apply low frequency 1V, 1Hz signal at the (+) input through the 1:1 000 000 divider (or 1:100 000 000 if necessary ).
4. Measure the AC component at the output (output swing should not reach the saturation region).
5. Divide apropriate values .
6. Check if the AC frequency and the capacitor were selected correctly:
a) Repead the measurement using the half of the original frequency (1/2 Hz)
b) Repead the measurement using the original frequency (1 Hz) and double the capacitor value.
The calculated gain should be the same for all measurements.
If they are not, then lower the frequency and increase the capacitor as long as you will get the same gain for different frequences/capacitors.