what are the first clues that charles and hal do not belong in the yukon ?

They are the true example of "greenhorns," men who know nothing except they think they can get rich quick.  They have brought entirely too many material possessions, they know nothing about how to load and run a sled with its sled team; they ignore the warnings of those who are more familiar with the Yukon environment.  They simply should have stayed back where they came from.