The outflow Valves can sometimes not be seen via a visual walkaround. I am a prior C141B Flight engineer and on that plane our outflow Valves were controlled by a Pneumatic relay via our control or touchdown switches. Their position was never checked visually as a preflight duty. Both because of location and type. However we could tell pretty quick on climbout that Cabin press. was with the aircrafts altitude by looking at our diff pressure gauge or by how it felt. Especially through 8,000 feet and up.