- Sudden weight loss (>10-15 pounds without a change in exercise or diet)

- Any kind of recurring pain, especially headaches

- Your poop is messed up

- Blood anywhere - coughing up blood, nosebleeds when you don't usually have them, blood in your poop or urine

- Mood or behavior changes (helpful if you have someone close to you who may notice this)

- Feeling fatigued even when well-rested

- Swelling or lumps anywhere

- Trouble sleeping

Which is a lot of stuff, but fortunately can be summed up as "If you feel consistently worse than you usually do, go see a doctor".

The key is to make your judgement of "I am indeed feeling consistently worse" within a couple of weeks, not a few months or even years while hoping it'll get better on its own. It's harder than it sounds - life is always busy and it's easy to tell yourself that your discomfort is mild and you can hold out a little longer. I waited too long to act on some bad heartburn and my few months of delay were rewarded with permanent damage to my lower esophagus...