All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
There are two signs of recognizing the end of the menstrual period, namely, the white discharge or dryness. Complete dryness is verified when the woman inserts a piece of cotton in the part of her vagina that is visible when she sits to relieve herself and it comes out dry with no trace of blood nor a yellowish or brownish trace on it. She does not have to insert the cotton into the inside of her vagina; rather, it is enough that she inserts it in the part that is visible when she sits to relieve herself, as has been mentioned above.
In case of doubting whether the period is over or not, the basic principle is that it is not over. Hence, if you used to perform Ghusl before knowing with certainty or believing that your period was most likely over, then the Ghusl is invalid. As for making up for all the prayers performed in that state of doubtful purity, the scholars held different views in this regard. Perhaps the most lenient opinion is the one held by Shaykhul-Islam Ibn Taymiyyah and those who concurred with him, that the woman is not obliged to make up for the prayers that she performed in such a case given her ignorance of the relevant religious ruling.
Allaah knows best.