You don't test ohms - you calcute how many ohms are needed to meet your current goal/requirement.
If you know the Forward Voltage, and you know the current you want, and you know the voltage of your source, then you can calculate the resistance needed:
(Vsourse - V(LED forward voltage))/current desired = resistance
So
(5V - 3.2V)/20mA = 90 ohm
I think both 91, 100 are standard, go with those.
20,000mCD is VERY bright, do not look directly at the light.
You may find you have to increase the resistance a LOT and only run at 3-4mA to make the brightness bearable depending on where the LED is being used.
(5-3.2)/.003 = 1.8K