Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services if cold exposure is accompanied by signs of hypothermia, such as confusion, unusual drowsiness, slurred speech, poor coordination, slow or shallow breathing, or loss of consciousness.
Move the person to shelter if this can be done safely, but do not delay emergency help.
Mild frostnip that returns fully to normal after warming may not require emergency treatment.
Get urgent medical assessment for skin that is frozen, hard, waxy, blistered, deeply discoloured, or still numb after gentle rewarming.
Significant swelling, loss of movement, severe pain, or frostbite affecting a large area also needs prompt care.
Do not wait for skin to turn black.
That is a late change and is not required for frostbite to be serious.
After a diagnosed injury, contact the healthcare team promptly if there is spreading redness, increasing warmth or swelling, pus, a bad smell, fever, or pain that is getting worse rather than better.
These can be signs of infection or another complication.
And for anyone, someone who:
Routine medical advice is still reasonable if symptoms recur, sensation does not completely return, or you are unsure whether the tissue froze.
Every sign above is quoted from the sofidex article on Frostbite. If you are not sure, err upward — it is always reasonable to be checked.