This question is in the context of being a passenger on a commercial airliner, where you're in the cabin and the cockpit door is locked.
In another question, someone posited that if an airliner had reduced its airspeed by 60%, he'd be quite concerned. I asked "How would he even know?"
How would a passenger on a commercial airliner, who is reasonably technically savvy but not a pilot, be able to discern airspeed?
This is old school. The person does not have any modern smart devices. Or does, but their battery ran down. (Not looking for a product reco for a gadget or app, there's another answer for that.) The person could probably improvise a plumb bob, stuff like that.
Would anything change if the passenger has a clear view of the ground, relevant maps with scales, and a stopwatch (but not any wind data)?