http://www.bt.cdc.gov/disasters/winter/faq.asp#signs
Keep in mind that some of the signs of hypothermia are associated with folks carrying out tasks involving movement. Someone such as ourselves riding a bike may not even notice the initial stages. Negative affects on our motor skills may not be evident until you need that quick reaction on the clutch, brake, shifter, etc. Check out the wind chill charts available in many places on the Web. When your riding at 60 mph, you are exposing yourself to 60 mph winds and it does not take much of a cool temp to drain heat from your body. There's only about 5 degree difference in you core body temp between life and death.
Bob's answer is, unfortunately, excellent and correct as far as it goes. On Search and Rescue missions we (are supposed to) monitor each other. Someone else may notice what the 'victim' himself doesn't; especially during the subtle, early stages.
I think the best solution for the solo individual (me on my commute this time of year, or a solo hiker, skier, etc.) is to have an inviolable routine that includes fairly frequent stops to warm up somehow. For a rider, that can be inconvenient at times, but really necessarry. I also recommend starting out dressed WARMER for the ride than you anticipate to be necessarry (we do the opposite for climbing or hiking or patrols because we know we'll generate a lot of heat from the intense physical activity). Better to err on the side of caution. If need be, after 15-20 minutes you'll just have to stop and strip off a layer. It also means you need someplace to put stuff. Admittedly, I violate that last suggestion myself on my daily commute, because I tell myself I just can't afford the time to stop, and it's only an hour's ride.
A few years back, I totalled my car from what I'm sure was a hypothermic-thinking poor decision. We'd been out all night looking for a lost skier, slogging through thigh-deep snow (that's how far we'd sink in even on snowshoes). Come morning I signed out, prepared to come back after a few hours rest to work with the dogs. I actually thought to myself, "it's cold, I'm tired, we have a warm ski lodge for base; I should sleep on the floor here for a bit before heading out...na, I'll be fine for the short drive". Part way down the mountain I nodded off behind the wheel and came to when my vehicle was airborne. I am absolutely convinced, as are my teammates, that I made that bad decision just because I was a little too cold. I'd seen others do it, but I fell into that trap anyway...
Ride safe out there! Stay warm, fed, and hydrated...
