Abdominal pain, heartburn and fatigue can sometimes be symptoms of stomach cancer. But they can also be the result of overeating, untreated acid reflux or insomnia.

So, how do you know which category your symptoms fall into and when to see a doctor?

“Most stomach cancers don’t cause any symptoms until they’re more advanced, and they can mimic indigestion or reflux,” says Paul Mansfield, M.D., a surgical oncologist who specializes...