How can I tell the resistance of an internal pull-up resistor? I have a SparkFun ESP8266 Thing Dev board, to which I want to perform a digitalRead() on one of the GPIO ports. I'm using D15, and I'm configuring it for input like this

pinMode(15, INPUT_PULLUP);


It does seem to work as intended, and I am able to pull it down by connecting it to ground via a 4.7kΩ resistor.

But how do I know what the actual "correct" resistor would be? I can't find any specs for the internal pullup resistor online. On a related note, I first tried using D0, but it didn't seem to have an internal pullup resistor, since even using INPUT_PULLUP didn't turn it on. 

Perhaps there are specs online and I'm just not able to find them, but is there any other way (measuring?) to find out which pins will work with INPUT_PULLUP and if so, what their internal resistance is?
io 0 and io 15 are with io 2 boot configuration pins of the esp8266. esp8266 dev boards have external pull-up on io 0 and io 2 and pull-down on io 15.