how can we read the exact potentiometer value?
You use a Wheatstone Bridge.
Wheatstone Bridge
A Wheatstone Bridge is a simple circuit that allows you to determine the resistance of a resistor with an unknown value. The circuit uses a voltage source, two resistors with known values, an adjustable resistor (potentiometer or resistor you swap out by hand), and a volt meter. You adjust the value of the adjustable resistor until the voltmeter reads zero. At that point, you use Kirchoffs laws to calculate the value of the unknown resistor.
Or, use a constant current source, run it through the potentiometer, read the voltage across the potentiometer, and calculate the resistance with V = IR .
There are lots of other ways to do this as well I'm sure.