Sometimes I see larger grit in the chicken poop but there is plenty of fine grit for them, they are just choosing the larger grains at times.
Any grit that is being passed to the poop is too small, that is why it was passed instead of retained... They need the larger insoluble grit that gets stuck in the gizzard and is used to grind up food, small sand like grit and/or grit that has been ground down below the optimal size is what is passed out of the gizzard and is mostly useless... Grit for adult chickens is not sand size or even that small, it's small rock size, about 5/16"-3/8" chunks for most adult chickens... Also the type of rock plays a role, as I said ideally they want insoluble grit for food processing, thus if you live in a region where there is a lot of limestone it's not optimal for food processing but it's a great calcium source... And if most of the rocks in your are 'rounded' and 'smooth' they are not going to be optimal for grits purpose...
Can a chicken get grit from free ranging, sure if they have enough range and the ground provides suitable gravel and stones for them, this is fully dependent on location...
I will state that for the literal few dollars it cost for a 50lb properly sized insoluble grit, it's cheap insurance they are getting what they need, and likely pays for itself over and over again due to better conversation rates of their feed since you optimized their grit...
To be clear, this is not directed at anyone particular just an observation over the years, it never ceases to amaze me that many will spend money on premium feeds and treats without hesitation or a second thought, but are reluctant to spend $5 on grit that will last for months... For most small backyard flocks a 50lb bag of grit will literally last years, or at least it should, since I offer it free choice to my 100ish birds 24/7 and only go through a few bags of grit a year...