What is the best way to safely verify the state of a digital pin on a live circuit by external means?
Think of the pin as a Thevenin source. In order to distinguish the four states you've enumerated, you need to determine two parameters: the Thevenin source voltage and the Thevenin source resistance.

In order to do this, you need to measure both the pin's voltage and current, and you need to do this relative to at least two different external references (e.g., Gnd and Vcc). Only then will you have enough information to derive the Thevenin parameters.