Acoustic analysis: How to assess voicing?

edit

I am in the process of analysing post-alveolar fricatives, and one parameter that is not always easy to assess aurally is voicing. I would like to have a look at them in PRAAT, but especially for these unclear cases, I am unsure how to interpret the F0 image in the spectrogram.

Is there any dynamic range threshold that would allow me to say, everything above X dB is voiced, everything below X dB is unvoiced? Or is it dependent on the volume of speech and the dB ranges in the surrounding, and therefore maybe more complicated?