Measure Voltage (Divide) on a current source
To measure the voltage across the resistor under test, you just measure it, like by connecting a voltmeter across it.

With 20 mA thru the resistor under test and 10 V max, the highest resistance you can measure is 500 &Omega;.  Even a crappy voltmeter will have so much higher resistance that it won't distort the measurement.  For example, let's say you're measuring a 500 &Omega; resistor.  According to your spec, it has 20 mA going thru it, so 10 V across it.  A 1 M&Omega; (that's really crappy) voltmeter will draw 10 &micro;A, which will distort the measurement by (10 &micro;A)/(20 mA) = 0.05%.  Is your voltmeter good to 0.05%?  I didn't think so.

Added

You have now shown a schematic.  The voltmeter is in the wrong place.  Put it directly across the resistor under test.