"There is a ritual method of testing whether menstruation has genuinely ceased, known as the hefsek tahara. To perform this, the woman takes a bath or shower near sunset, cleaning herself everywhere, and after a few minutes wraps a special cloth around her fingers, and swipes the vaginal circumference to the greatest depth she can manage; if the cloth only contains discharges that are white, yellow, or clear, then the menstruation is considered to have ceased, but if it is bright red it indicates that menstruation continues, and if it is any other colour it is subject to further inquiry, often involving the consultation of a rabbi. The ritual requires that the cloth used to perform this test is first checked carefully to ensure that it is clean of any marks, colored threads, or specks; the cloth itself can be any clean white cloth, although there are small cloths designed for this ritual, known as bedikah (meaning checking)." "The extent to which the rabbinical and biblical laws of niddah are followed differ. Sephardic women, even apparently secular ones, are reputed to follow them strictly; on the other hand, the laws tend to be ignored by secular Ashkenazi women. These laws are strictly followed in the more traditional Orthodox Judaism, who view them as serving to elevate the physical to the highest spiritual level; while the progressive movements, such as Reform Judaism, often interpret them as a mechanism of rediscovering female spirituality; the progressive movements had previously abandoned practice of the niddah laws, but some groups are now re-adopting them to varying extents. Knowledge of the regulations concerning niddah is essential for rabbis in Orthodox Judaism, and without it they cannot attain ordination; in addition to the Talmudic regulations, Orthodox Judaism views the Shulchan Aruch as being particularly authoritive on these matters." Source and further information: http://en.wikipedia.org/wiki/Niddah