Signs of purity from menstruation should be checked before prayer time ends
19-9-2016 | IslamWeb
Question:
If a woman is in the last day of menses when it is slightly yellowish till she sleeps at night, should she wake up for the Fajr specifically to check if it has ended, or should she check when she wakes up in the morning and then do Ghusl (ritual shower taken in the state of major ritual impurity)? May Allaah reward you.
Answer:
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.
It is incumbent on the woman to check if her menses has ended before the end of the prayer's time, and she is not obliged to wake up before Fajr, nor is she obliged to check at other than prayer times. It is enough for her to check before going to bed and then act upon the state that she is in, namely that she is still menstruating. However, she is obliged to wake up before the end of the time for performing the Fajr prayer (i.e. early enough before sunrise) in order to check for the signs of purity lest she attains purity and misses the obligatory prayer. Ad-Dardeer 
wrote:
"It is incumbent on the menstruating woman to check for the signs of purity before sleeping at night so as to know whether she can perform the night prayer or fast (on the following day). The basic principle in this case should be the continuity of her current state (menstruation). When the Fajr prayer and other obligatory prayers become due, she is obliged to check for the signs of purity at leisure (this obligation is Muwassa' [flexible], i.e. it can be performed within its designated time whenever one wishes whether at the beginning, in the middle, or at the end) until there remains only enough time to make Ghusl and pray before the prayer time ends. In that case, it becomes obligatory to check for purity right away (this obligation is Mudhayyaq [rigid], i.e. it must be performed immediately on its designated time without delay)." [End of quote]
Hence, it is impermissible to delay checking for purity until the sun rises, as understood from the quote cited above.
Allaah knows best.