You have to use main arteries which close to the surface point. Like armpit and groin. Because heat on the body transporting with blood. Especially with arteries. Maybe you can use scalp because in that area have much more capillary.
Peter's answer may use but; arteries getting smaller when cold. if A. carotis can't send enough blood to brain, person may faint. It may damage to the brain. Maybe it will not be a problem if done at short intervals.