Rulings pertaining to Menses
By Shaykh Saalih Al Fawzaan:
The question here is: Since the sister experiencing Istihadah is legally considered pure (to perform acts of worships), how can she distinguish between menstruation and bleeding of Istihadah when her bleeding continues all the time?
There are three cases for the sisters that experience Istihadah
When a woman used to have a stable, regular menstrual period (i.e. having certain duration of menstrual period) before she had Istihadah(irregular bleeding). In such a case, a woman can distinguish between both states relying on her stable menstrual period (i.e. the bleeding that contradicts the duration of her menstrual period is regarded as that ofIstihadah). Thus such a woman can wait until her usual period ends, then deem any other bleeding as Istihadah.
When a woman does not have a regular menstrual period but her bleeding is always distinguishable; sometimes she bleeds malodorous (with a scent), thick, and black blood (i.e. having characteristics of menstruation blood), and some other times she bleeds red blood which is neither thick nor malodorous. Such a woman is to consider the former kind of blood as that of her menstrual period, during which she gives up prayer and fasting, in regards to the other kind of (red) blood is that of Istihadah, in this period she can perform prayer and observe fasting, for she is considered ritually pure in this case.
When a women has neither a regular menstrual period nor a distinguishable kind of blood (e.g. the perplexed woman). Such a woman must follow the prevalent womanly menstrual period (i.e. average of six or seven days a month) and consider whatever discharge after this period as Istihadah.
To sum up, we can say:
A woman with a stable, regular menstrual period distinguishesIstihadah according to her habits (menstrual period she used to have).
A woman with a distinguishable blood depends on her own judgement and her ability to distinguish between her menstruation and Istihadah.
A woman that has neither (a regular period nor distinguishable blood) is to consider six or seven days a month and then take a ritual bath (ghusl).
These above mentioned rulings combine the three rules stated by the prophet (sallallhu ‘alayhi wa sallam) concerning a woman in a state ofIstihadah.
Al-Mulakhas Al-Fiqhee by Shaykh Salih Al-Fawzan, Volume 1 1.13