Here's an interesting list of symptoms of Stage 1, 2 and 3 hypothermia: http://www.jonesactlaw.com/library/hypothermia-at-sea-a-major-risk-for-maritime-workers.cfm Yes, it's from a law firm... think they can be trusted?

The touching-thumb-to-little-finger is the simplest way to determine if someone is becoming hypothermic. If you're traveling with people you don't know reasonably well (or at all), and ask someone if they're too cold and they say they're all right, it's a reasonable request to ask them to prove it this way. It isn't like you're asking them to disrobe so you can stick an 8" needle/thermometer into their liver.

It seems to me that if you think someone is hypothermic, they probably are. You'll have to use your own judgment, not that of the victim.

Sue