That's a difficult question to answer because there are so many different kinds of cancer, and many have their own symptoms.
Some of the more general things to look for are a sudden unexpl;ained weight loss (that is, you haven't been exercising more or dieting), extreme unexplained fatigue (but fatigue can be a symptom of MANY things, including depression, insomnia, sleep apnea, etc.).
If you have some symptoms that concern you, your best bet is to talk to your physician about them. And, of course you can always post here with more specifics and people will try to give you some guidance.