Go and get yourselves on an Aviation Medicine course, in the Baro chamber. You will learn alot about your own personal limitations indications.
Most won't notice the effects of hypoxia - because one of the effects is "Feeling Great". And this is whilst sitting in the chamber and actually expecting it.
Let alone bimbling along at 90kts trying to do your TAS calculations and watching your VSI, fumbling with the piddle pack because it is soooo cold. You'll never see hypoxia coming with such a high workload.