Screening tests can help detect cancer early, but you should always be on the lookout for symptoms. The American Cancer Society has developed this short list of symptoms that can help you catch the disease at an early stage when it is most likely to be cured. You should see your doctor if you have any of the following symptoms:

• Unexplained, persistent fatigue;
• Weight loss;
• Changes in bowel or bladder habits;
• Pain that doesn’t go away;
• Unusual bleeding (even light bleeding after menopause or inbetween periods);
•  A lump in the breast or elsewhere on the body;
• Indigestion or difficulty swallowing;
•  A noticeable change in a wart or mole;
• Persistent cough or hoarseness. 

This is a general guide. The vast majority of these symptoms are caused by non-malignant conditions, and cancer can also produce symptoms that do not appear on the list, but it is good to pay attention to your body's signals and discuss them with your doctor when you encounter them.