Factual error: Several mistakes about the clock in Jigsaw's workshop: around 300 years ago the first pendulum clocks were built. They were completely different by size and design than this one, so the claim that it is so old is completely wrong. Real pendulum clocks like the one seen in the movie never have a second hand from center. You can also see by the movement of the second hand that it is a modern, battery operated quartz movement. This also means that the ticking you hear is not created by the clock itself but was added later. (00:38:40)
Factual error: It would not be possible for the scalping device to scalp Brenda's hair in the manner that it does. Hair is too thin to withstand that much pressure when being pulled and would rip long before she began to bleed profusely. At worst, she would likely be bleeding a little bit in a non-fatal manner from having her hair yanked after it ripped.
Answer: Realistically there is no way he could have known. The writers were just working backwards from the plot point on this one.
Phaneron ★