How can you tell if someone has wet gangrene?

Well since it has something to do with blood supply, and it can be caused by smoking. So maybe if they look pale, or since they smoke a lot. 

Well I looked it up. I honestly don't know if this is the right answer you are looking for :
It almost always involves an infection. Injury from burns or trauma where a body part is crushed or squeezed can rapidly cut off blood supply to the affected area, causing tissue death and increased risk of infection. The tissue swells and blisters and is called "wet" because of pus. Infection from wet gangrene can spread quickly throughout the body, making wet gangrene a very serious and potentially life-threatening condition if not treated quickly.

Tobacco would be the answer . good luck!