The advice not to look is not a halachic requirement, but a recommendation to help women avoid sometimes complicated niddah questions.
Stains found on colored undergarments do not make a woman niddah, whether she sees them or not. Blood found in the toilet bowl also may be treated as a stain that does not make a woman niddah. So too, we follow the halachic position that blood found on toilet paper may be disregarded if there was an interval of at least a few seconds between urinating and wiping. (For more information, please see our articles on “Stains” and “Toilet Paper“.) Any of these can help you check whether your period has started, without risking becoming niddah unnecessarily from staining.
Please note that when a woman has staining that does not make her niddah, we usually recommend that she refrain from relations for a day, as a precuation against a flow beginning during relations. This is a precaution and not a strict halachic requirement.
Please write back with any further questions.