The FAA puts on workshops around the country with a portable reduced oxygen training enclosure (PROTE). You sit in the enclosure (looks like a sealed vinyl tent), they reduce the available oxygen and simulate hypoxia. You've got a clipboard with some basic math problems, a maze to trace, etc. The trainers continually engage you for 3-4 minutes as you slowly get more hypoxic.

As a pilot, it was eye opening to see first-hand what happens to me when experiencing hypoxia. The trainers were talking to me, and I was replying, but was unable to tell them what 17 minus 4.5 was. My pulse oximeter was in the low 70s. Two sips of oxygen from a mask and I was right back to normal. I learned that my first symptom (the clue that something is really going wrong in the cockpit) is tunnel vision.