| You can use a multimeter/voltmeter to get an approximation. However, that won't show you if there's very short term dips in the supply due to impedence along the way.
As for the motherboard monitoring chip, typically they have a resolution of 0.016 of a volt for voltages under 4.096V (as they use an 8bit ADC). That means they can only measure the voltage to the nearest 0.016V. However, 12V is over 4.096V so there's an external circuit that divides the voltage down.
This lowers the resolution of the monitoring chip, so the 0.07v difference you're seeing may be nothing more than the smallest voltage change the chip can determine. The real values could be 11.80 (which the chip might round to 11.77) and 11.81 (which the chip might round to 11.84).
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