Purity: Determining the Type of Istiḥāḍah
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 402:
If a woman does not know which type of istiḥāḍah she has, then based on obligatory precaution, she must examine herself when she wants to perform prayers. For example, she must insert some cotton into the vagina, wait a little while, and then take it out. After discovering which of the three types of istiḥāḍah she has, she must do the things that have been instructed for that particular type. However, if she knows that until the time she wants to perform prayers her istiḥāḍah will not change, she can examine herself before the time for prayer has set in.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)