What do I look for with frost bite?

[Jo Anne]

This is my first winter with chickens, I've read threads about frostbite. I have 4 Rir in an A frame tractor. Picture in my avatar. They have a tarp over the whole tractor, straw in their roost, and closed ramp.
Just in case, I want to be prepared also since the wind chill will be in the negative 2 digets.:

 

[Judy]

Frostbite is prevented primarily by making sure the humidity they put out in their breath and poop can escape from their coop during the night. There needs to be a substantial opening at the hhigh point of the coop, as their warmer air rises.