The classic field identification method is to look for the "umbles." This is a term that comes from the wilderness medicine community and refers to a change in a person's level of consciousness and physical coordination. In essence, if you see someone stumbling, mumbling, bumbling, and fumbling, then he or she may be experiencing hypothermia. Conversely, the umbles are associated with heat-related injuries as well.
Keep in mind that most cases of hypothermia happen in 50 degree F weather and not just in the depths of winter. Many dayhikers who succumb to it are dressed improperly, wearing cotton clothes. Wet cotton will contribute to hypothermia as it wicks your body heat away from you and fails to insulate, unlike wool and fleece garments. So, avoid the jeans, sweatshirt, and other 100-percent cotton fabrics.
If you are experiencing hypothermia, or notice someone in your hiking group with the umbles, do something about it immediately and prevent it from getting worse. Make a fire, get out of the wind, change into dry clothes/footwear, or get into a tent and sleeping bag, and otherwise eliminate the conditions that are creating the problem. Then, stoke your internal furnace with some high-calorie, high-fat foods.
My usual cold-weather recipe for when I start to get chilled is to have a cup of hot cocoa with a tablespoon of butter in it. I carry a thermos with this brew on my winter day hikes. The combination of fat and sugar helps to amp up the metabolism.
Hypothermia is the number-one killer of people in the outdoors, so dress properly to deal with wet, windy conditions, bring high-energy foods, stay hydrated, carry at least three fire-starters, and keep an eye out for the umbles.