How to measure voltage of a floating GPIO (Raspberry Pi)
One simple way is to measure the voltage to ground and then measure it to Vdd (which is 3.3V on the Raspberry Pi SOC). 

Since most voltmeters have something like 10M input resistance you've effectively got a resistor across the meter input. 







simulate this circuit &ndash; Schematic created using CircuitLab

If the meter reads about zero both ways then it is relatively high impedance (if there was 10nA leakage it would read 0.1 Volt). Leakage in the high impedance state is typically very loosely specified  but it is normally extremely low at room temperature, so this method does work. 



If it reads non-zero but both voltages add up to Vdd then it is probably fighting another output or is being switched at a high frequency (an oscilloscope will tell you the difference). This is the 4th combination of measurements, the first two are obviously high and low.