Blood in stool has a strong effect on the odor, so you'd probably notice it. If your stool smells dramatically worse (or at least different) than usual, in a ma

Blood in stool has a strong effect on the odor, so you'd probably notice it. If your stool smells dramatically worse (or at least different) than usual, in a maybe metallic way, that's how you know. And "unusual" usually means unusual for you. So if you don't change your diet or habits, but suddenly get a ton of gas, fatigue, need to go to the bathroom way more often or have a different experience going to the bathroom, it's worth mentioning to do your doctor.