How can she know that her menses has ended, as that may happen and then be followed by yellowish or brownish discharge without her realising?

Firstly:

A woman may work out whether she has become pure [that is, her menses has ended] by one of two signs: either emission of the white discharge, or complete dryness of that place.

Secondly:

A woman has a menstrual cycle that she is familiar with, and she can recognise menstrual blood, and when the period starts and ends.

So when the time for her period ends and the time for her becoming pure is close, she should check to see whether she has become pure before she goes to sleep, and at the times of the daily prayers. So she should check before she goes to sleep and at the time of each prayer, to see whether she has become pure or not. If she sees the signs of becoming pure that she recognises in her own case, then she has become pure, and if she does not see that, then she is still menstruating.

It says in Mawahib al-Jalil by al-Hattab al-Maliki (1/372):

Malik said: A woman does not have to keep checking to see whether she has become pure at night and at dawn; rather she should check when she wants to sleep and when she wakes up for the dawn prayer. She should also check at the times of prayer… End quote.

The woman should not hasten until she sees the sign that she has become pure [and her menses has ended]. When she sees that, she should do ghusl and pray and fast.

Shaykh Ibn Baz (may Allah have mercy on him) said:

Becoming pure from menses happens when there is no trace of blood. So if the woman cleans herself with a piece of cotton or the like, and sees no trace of blood on it, then she must do ghusl, even if she does not see the white discharge. As for the white discharge, it is a white fluid that happens to some women at the end of menses; some women see a white discharge, which is a sign that menses has ended, and some women do not see that. What matters is that it should be clean, so that if a woman wipes herself with a piece of cotton and the like and sees that it is clean, with no yellowish or brownish discharge, then she should do ghusl,  even if she does not see the white discharge."(Fatawa Nur ‘ala ad-Darb  5/411).

Ibn ‘Uthaymin (may Allah have mercy on him) said:

The basic principle is that menses is ongoing until the woman is certain that she has become pure [and menses has ended]. The same applies to anything that already exists; the basic principle is that it still exists until there is certainty that it is no longer there."(Jalasat Ramadaniyyah  5/17).

Thirdly:

Allah has not imposed any hardship on people in religion and the rulings of Islam are easy, with no difficulty or hardship in them; rather they are all easy.

So at the time when her period is due to end, a woman only has to check herself before going to sleep and at the times of prayer. She does not have to check constantly whether she has become pure or not.

Usually, if she is in a meeting or is busy with something, at the time of each prayer she goes to pray. So at those times she can check herself as described above.

If a woman makes a mistake in determining the time her period ended, based on what she thinks, she is not sinning, because Allah, may He be exalted, says (interpretation of the meaning): {And there is no blame upon you for that in which you have erred but [only for] what your hearts intended} [al-Ahzab 33:5].  But if she thinks that she has become pure [and her period has ended], and she prays and fasts, then she realises that she is still menstruating, she must stop praying and fasting until her period ends, and she must make up any obligatory fasts that she did during those days, because it has become clear that they were not valid.

If she does not pray because she thinks that she has not become pure, then she realises that she was pure [not menstruating], then she must make up those prayers, to be on the safe side, but if she does not make them up, then there is no blame on her, in sha Allah.

See also the answer to question no. 45885 .

Doubts and speculation carry no weight, so thinking that perhaps she became pure without realising is of no significance, because Allah does not burden any soul with more than it can bear.

Regarding the questioner’s words, “perhaps her being pure only lasted for a few minutes, then when she checks to find out whether her menses has ended or not, she sees yellowish or brownish discharge, so she does not know whether this brownish or yellowish discharge was part of her menses or came after her menses had ended,”

she is being too hard on herself and there is no need for that. If we were to decide that there is the possibility of the woman becoming pure for five minutes only, then that would mean that she would have to check herself every five minutes to see whether her period had ended or not.

For more information, see the answer to question no. 201425 .

And Allah knows best.