Smell them. A good indicator as to whether or not a person smokes is simply by smelling them. Frequent smokers are so accustomed to the smell of their product they are not aware that the scent of said product. Their noses have long since adjusted to the smell.
There are many ways, some of which are: how lethargic the person is, how dilated are their pupils, their coordination, their speech and attitude, their appetite and their overall mood.
