How to tell if fog is made out of water droplets or ice crystals?
This doesn't aim to be a complete answer, but after putting a juicy bounty at Aviation SE on the question "Can pilots tell if a cloud or fog is made out of water droplets or ice crystals when flying through it?", the answer seem to be: No, they can't. And all the things to look at to detect ice clouds doesn't seem to be useful if you don't have a plane at hand. There is nothing like "there is a special refraction pattern around the lights on the wings", or such.

So far it seem like ice and water clouds are indistinguishable.