In Samurai Champloo, how in the world did Sara realize her son was dead while talking to the pinwheel guy?
She had some sort of extrasensory perception, which is proven by the fact that she was blind, but still proficient in fighting. Also, she tells Fuu that she is able to sense things she cannot see, and she can sense Momo hidden in her clothes. When she fights Muugen, she tells him she can sense his anger, meaning she could somehow feel what other people feel and their states of mind.
Given the above, and the fact that she realizes that her son is dead when a pinwheel stops, I can only conclude that the stopping of the pinwheel made her realize some state of mind/feeling coming from the pinwheel merchant (something she was not aware of before), which in turn made her realize her son was dead.
Although this is speculation, it is the only thing that seems plausible to me. However, the stopping of the pinwheel may have some deeper meaning that I am not aware of.
Though this answer is about 4 years late but I just got done rewatching Samurai Champloo recently so here it goes! So the little shrine with the statue in it that the pinwheel guy is sitting behind is called a Jizo statue. Which is a shrine women who have had miscarriages, abortions, or grieving go to pray. That being said I also believe that she realized that her son was dead when the pinwheel stopped but that the statue somehow had something to do with it. At the very least it was symbolic as they show the statue itself a few times.
She mentioned to Muugen that being blind, she adapted the ability to sense the aura of individuals as each is significantly different. The pinwheel suggested to the viewers the sons dead. But it was the boxes the man was travelling with that actually gave it away. Boxes the size of a human head. Sara was able to sense her son's aura and that is how she knew her son was dead. I guess carrying decaying heads around wasnt as frowned upon back then. Still fucked up tho