Factual error: James Graham, 4th Marquess of Montrose, became 1st Duke of Montrose in 1707, six years before the film is set, although the film refers to him throughout as a Marquess.
Factual error: When the English raid Rob's farm and shoot and kill the cattle, the cattle fall over on their side like toys. Four legged animals shot like that collapse straight down as their legs fail under them.
Factual error: Rob Roy is asked by Montrose to help frame Argyll as a Jacobite. This is the sort of thing the highland aristocracy did all the time, but no-one would have tried to frame the Duke of Argyll. He was the government's main man in the highlands, to whom the framers brought their (real or fake) evidence against their rivals.
Factual error: John Campbell, 2nd Duke of Argyll was 33 at the time the film is set; Andrew Keir, at 69, was more than twice the age of the character he played.
Factual error: At one point, Rob Roy refers to the Duke of Argyll as "Your Lordship". Dukes are only ever referred to as "Your Grace" and he is addressed correctly (by Rob Roy and others) throughout the rest of the film.
Answer: Cow patties leaving the cow and plopping on the ground leave a distinctively odor and start to harden up after a certain length of time. It isn't to hard to speculate that, since these are cattlemen by trade, they would have a systematic way of knowing how long it would take for a cow pie to harden & lose it's odor. Example: they would know that a cow pie left out for 2 days would lose half it's odor & become semi-hard but still have a mushy center.
CCARNI ★