Three steps:
Numb, red, swelling, uncomfortable when warming
Warmth after frostnip. Mottled skin. May blister after 12-36 hours, pain when warming. Get to urgent care.
capillary refill takes ~10 seconds
Skin looks blue/gray, no pain or sensation. It's down to the bone, skin has died.
Warm, not hot water. For anything past frostnip, get professional help.
don't rub the skin of a frostbite victim
Don't use a fireplace or other radiant heat