How could you find the resistance of a resistor?

You can find the resistance of a resistor by applying a known voltage across it and then measuring the current through it. By Ohm's law, resistance is voltage divided by current. You need to be careful to not pass too much current through the resistor, because any significant power will cause heat which will bias the results.

Resistors are also color coded with resistance and tolerance bands. Color charts are available on the internet. They can also be measured with a VOM.