Non smoker, rather active, good health.....may be fine.
Couch potato, smoker, heart issues, etc., may be in bad shape walking in Denver, let alone flying higher....yes, I have had to run through the Denver airport from a late flight trying to catch a connecting flight.....sucked carrying a heavy bag...
YMMV......

Yes, finger probe (I believe under $100US) will basically tell you how you're doing on O2.