How can you tell if your cold hands have frostbite or not yet?

Warm & observe. There is a fine line between chillblains and frostbite. If you suspect this, you should first institute gentle and slow rewarming of the effected part and monitor the area for tissue loss or death manifected by a loss of sensation or vascular supply.
White, numb. The earliest signs of frostbite are white, red, or yellow patches of skin, followed by numbness. In more severe cases blisters develop 1–2 days later, then become hard and blackened (but usually appear worse than they are).