Areas susceptible to frostbite are the tips of ears, the tail, and pads of the feet. Additionally there will be a lack of pain or sensation of the affected area or it may be extremely painful when the area begins to warm up. In the event your dog is cold, the first thing it will typically do is try to huddle with humans or other dogs. If your dog is seeking shelter or climbing into your sleeping bag, I would consider this a good indication that your dog is cold. Make sure the dog is drinking plenty of water as this will help to maintain body temperatures.
Booties are another excellent way to reduce exposure and heat loss.
In most cases cold sets in faster when activity is reduced. This is the time to be most aware of the threat of cold. Yes, CPR is a very real solution. I know of one dog that was successfully brought back to life after her heart and respiration stopped from smoke inhalation in a house fire. The procedure is different then the procedure used in resuscitating humans and the worst time to learn CPR is when you need to perform it.