Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Call emergency services or go to an emergency department if:
The joint or limb is visibly deformed
A bone is visible or there is a deep open wound
The area beyond the injury is cold, pale, blue, or numb
You cannot move the fingers or toes beyond the injury
Severe swelling or pain is increasing rapidly after a major injury
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Seek prompt medical assessment if you cannot put weight on the limb, cannot use the joint, or have marked tenderness directly over a bone.
You should also be assessed promptly after a forceful injury, if you heard a crack, or if the joint looks out of place.
3
Does any of this apply?
Book an appointment
Routine follow-up is appropriate when symptoms are not clearly improving after several days, recovery takes longer than expected, or the joint remains stiff, weak, or unstable.
Repeated sprains also deserve assessment because targeted rehabilitation may reduce further injuries.
The article also says
Get medical advice if pain or swelling is getting worse rather than better, or if bruising is extensive.
Redness, unusual warmth, fever, or drainage from a wound also needs assessment because these features are not typical of an uncomplicated sprain.
Every sign above is quoted from the sofidex article on Sprains. If you are not sure, err upward — it is always reasonable to be checked.