Irregular bowel movements, cramps that I couldn't attribute to what I ate, inexplicable fatigue. Eventually it was a bright red blood in stool which made me force my doctor to take it seriously.
https://www.cancer.org/cancer/types/colon-rectal-cancer/dete...
At least in the US, I've found that if you're paying cash up-front, you can shop around for what you want, without referrals, and without this kind of hesitation.