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Well, you can measure resistance is by applying a known current and measuring the voltage across the resistor; you know, like an ohmmeter. Or, you could use a voltage divider. Or a Wheatstone bridge. But I don't know of any way you can do this without some external circuitry. Maybe you can explain your circuit more, and why you want to measure this resistance.
Or, you could do something like use the resistor to adjust a voltage that controls P and I, and just measure that voltage.