Primary lung cancer (cancer coming from lung tissue) is rare in dogs, so the odds are very high that a tumor seen in the lungs is a metastasis from a tumor somewhere else. This is not always the case but it is a reasonable presumption. So in most cases, just seeing a tumor in the lungs is reason to suspect metastatic cancer.
Lung cancer in dogs

Since there have been little or no studies done on lung cancer in dogs, it's my own personal opinion that the refusal to adopt a pet out to a smoker is based solely on personal bias. An ureasonable bias that would rather see an animal put to death or live the rest of it's life in a cage rather than allow it to know love and companionship that all pets deserve!

No dog has ever died simply from having a stinky coat!