I use talcum powder. I also tie unwaxed dental floss to my bow, and stuff my pockets with with milkweed pods every time I see them.
Heres my thoery:
A) the wind is everything
B) for pointing out the general, predominant wind, the dental floss works great.
C)For watching the wind in a little more detail...thermals, etc., the talcum powder is awesome, but it disapates within about 10 yards. Also very hard to see in mist or snow.
D)The milk weed pods are helpfull in watching long distance patterens. Say your in a tree at a saddle between 2 swammps....I have a spot just like this....often the wind looks to be straight out of the west, but the pods show me that once they drop down to swamp level, the current takes a hard north direction.
Anyways....thats my theory. To answer your question, talcum powder, but the blue chalk idea maeks more sense.