A woman becomes niddah when blood leaves her uterus through the cervix. It does not matter how much or how little blood has come out, by Torah law even the smallest amount of uterine bleeding will make a woman niddah. Biblically a woman becomes niddah only if the discharge was one of four colors of red or black. However, already in Talmudic times, the ability to distinguish these four shades of red from other shades was lost. Therefore, a woman becomes niddah from a discharge that is any shade of red. A discharge that has no trace of red, for example a white or yellow discharge, does not make a woman niddah.
The Torah requires that a woman have a hargasha, a feeling of the blood leaving the uterus. Without a hargasha she only becomes a niddah derabanan. There are three types of hargasha. Contemporary Poskim agree that a woman becomes niddah when she gets her period, even if she did not have a hargasha.
A woman who did a bedikah (internal check with a cloth) and found blood on the cloth becomes niddah. This is different than a ketem because when a woman does a bedikah, she might have had a hargasha and confused it with the feeling of the cloth inside of her. It should be noted that it is best to avoid unnecessary bedikot. Even if one is concerned that blood may be coming out need not do a bedikah, so as not to cause oneself to be a niddah for no reason.
A woman who used the bathroom, and when wiping herself found some blood, should consult a rabbi. It is highly recommended to accustom oneself to not look at toilet paper after wiping. This will help avoid many questions.
If a woman finds blood on herself, her husband, or on bedsheets immediately following intercourse, she should consult a rabbi. This is similar to what was stated above, where we are concerned that she confused the hargasha of niddah due to the nature of intercourse.