What is health insurance pre existing condition for cancer without a previous diagnosis?

Lets say an uninsured person has cancer but has not been diagnosed and has no record of seeking treatment for cancer . They then get an individual health plan (not through work) , uming they qualify and are now covered, the next day they see a doctor and the doctor diagnoses them with cancer? Would they be covered,, or would the insurance company still consider this a preexisting condition, because the doctor can guess how long the cancer has been in the system and if he says it was there before the date they applied for insurance, then its preexisting and therefore not covered? If so is there a waiting period that you need to wait in order to get diagnosed,, for example you need to wait 6 months after being covered before you can get diagnosed,,,but is it also relevant to how old the doctor says the cancer is?,,so even if you wait 2 yrs after having insurance but the doctor says your cancer is 5 yrs old then its preexisting and not covered? - Q????????::::::: What is the best way or policy to get to make sure you will be covered for any conditions a doctor may find once you get insurance and go in for a check up??