For me, this is largely from experience. I do recall seeing a poop thread, but I am not sure where it is. LOL All I know, is the cecal poos are all too often! Haha. I brood chicks outside for this reason.
I've never gave mine anything, but there are tons of types of chicken poos... regular ones, watery ones, cecal ones that will stink up the brooder. Their poos change greatly with diet.