If u had cancer starting would you know it right away? How do they check for cancer. Blood work show u have cancer?

Cancer screening. There is no magic blood test or scan to screen .There are guidelines for screen common cancers. Like mammogram starting age 40 or earlier if strong family history. Regular pap smears , colonoscopy starting age 50 or earlier if family history. Psa test in men at age 50. Most cancers diagnosed on screening or when someone has symptoms and then more test are orderd.