How would you know that you have lung cancer?

Imaging and biopsy. Most lung cancers are discovered by imaging studies such as chest x-rays or ct scans. These will show mass lesions that should not normally be present. If such a lesion is detected, then it is usually sampled by needle aspiration or bronchoscopy, and cells or pieces of tissue are studied in a laboratory under a microscope to decide if a malignancy is present and to decide what type the tumor is.
X-ray. Really look, listen, feel, test. X-ray or ct can be first notice even without symptoms. Proof is biopsy or removal.