How can people know they have cancer before seeing a doctor?

Depends Cancer type: This is a question that a lot of people ask. It is often difficult to tell if there is a cancer. However, certainly if there are external signs like lumps or masses that are unusual (ie. Melanoma or breast), medical attention should be sought. In the brain, patient will have neurological symptoms like seizures, weakness etc. In the gut, nausea, loss of appetite, weight loss.