Measuring resistance conceptually and in code?
you cant control current output to a pin BUT you know a pin output 5V (if using a 5V a arduino, 3.3v in other case).

Then what you need is a know resistence resitor, so you can set up a voltage divider and then read the resulting voltage with an analogRead. That give you a 10bit precision value that you can easily translate to a voltage, and then use the formula on the linked page to find R1 or R2 (depends on how you wired up your know resistor)