Quick answer
What are the signs that nifās has ended?
Nifās has ended when a sign of purity appears: the qaṣṣah bayḍāʾ (a clear or white discharge) or complete dryness. Once observed within the 40-day window, ghusl becomes obligatory and worship resumes.
The signs that nifās has ended are the same two signs of purity that end ḥayḍ: the qaṣṣah bayḍāʾ — a clear or white discharge resembling plain water — or complete dryness, confirmed by a kursuf (cotton) check showing no trace of color.
Either sign is sufficient. Once you observe purity, your nifās is over even if fewer than 40 days have passed. You perform ghusl and resume prayer, fasting, recitation, ṭawāf, and intimacy.
Light brown or yellow discharge that appears after a clear sign of purity is generally not counted as nifās, drawing on the same principle as Umm ʿAṭiyyah's report regarding discharge after the time of purity. If genuine bleeding returns within the 40 days, the majority treat it as a continuation of nifās.
If you reach day 40 still bleeding, nifās ends by the maximum: perform ghusl, resume worship, and treat any further bleeding as istiḥāḍah. Noor helps you recognize these signs and tells you when worship resumes.
This summary draws on the established positions of the four Sunni madhhabs. For your specific situation, consult a qualified scholar.