Hi,
I see two methods for measuring an R:
1) Measuring V and I and applying Ohm´s law.
2) adding a known resistor and measure V across the known resistor and measuring V across the unknown resistor. Then R_x = R_k * V_x / V_k
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In a live circuit, you can not know how the excitation signal looks like. It may be DC, AC or pulsed ..it may be current or voltage.
Any added circuitry, like voltmeter, amperemeter, resistor may corrupt the measurement.
Thus none of the above methods is secure.
I reccommend to analyze the excitation signal first.
Klaus