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?