Purity: Distinguishing Ḥayḍ from Hymen Bleeding
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 446:
If a woman experiences bleeding and does not know whether it is ḥayḍ or bleeding caused by her hymen tearing, she must examine herself; i.e. she must insert some cotton in the vagina, wait a while, and then take it out. If she finds that blood has stained only the sides of the cotton, it is bleeding caused by her hymen tearing, but if it has reached all parts of the cotton, it is ḥayḍ.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)