Checkung nevus. Early on means, if you notice them changing size and color - have them checked and maybe removed. No spreading skin cancer as a result

Breast cancer: feeling knots, etc. are warning signs.

Lung cancer: pain

Brain cancer:

certain types of headaches can be ignored with pills, or checked

etc. etc.

I am not aware of any cancer, which has no warning signs. But yes, most if not all signs are not a guaranteed test result. And in fact, it probably does not helping many people freaking out, when they notice warning signs, caused by harmles other causes. But doing so in a rational manner definitely increases your chances of early spotting.