The O2 saturation monitors work by shining a light through your finger, usually a laser. Fully oxygenated blood is one color, less than fully oxygenated blood is another color. The meter reads the absorption of the light in those wavelengths, compares them, and spits out a percentage. Since it is only making a RATIO of The two absorptions rather than caring about absolute absorption it doesn't matter how much is lost to fingers of different thicknesses- it works just as well on thick fingers as on thin ones. (And, actually, I think you can make them that use light reflectivity rather than absorption, too.)
But they don't work well through nail polish. Yeah, I'm looking at YOU, Dave. :)
Needless to say, I use them every day. And, an unacclimated person in Colorado Springs probably walks around just fine with an O2 Sat of 94%. It is hard to pick a cutoff for what is clinical hypoxia. By definition it is "clinical" when you have symptoms, not at some specific number. So, frankly, a mini mental status exam is probably more useful in the situation we're talking about. Of course, if someone was loopy and their sat is 98% then you've probably rules out hypoxia as a cause. Check their pack for empty bourbon bottles.
They are fun to play around with, though. And, notwithstanding what I just said, if I found someone with a sat of, say, 70% I would certainly take note...