What you're doing here is making a potential divider - two resistors, one known, one unknown.
By measuring either the voltage, or the current, you can easily calculate the value if the unknown resistor - simple ohms law.
It's simple enough maths - BUT if you make the known resistor the same value as the uknown one, you don't have to calculate anything at all, just read it off.
Easiest way is to use a potentiometer, adjust it for half the original signal, then remove it and measure it's resistance with a meter,