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 now if the sting is followed by:
Trouble breathing, severe wheezing, or swelling of the face or throat
Fainting, collapse, confusion, or unresponsiveness
Chest pain or a very abnormal heartbeat
Severe widespread weakness or repeated vomiting
Rapidly spreading swelling or severe symptoms beyond the injured limb
No normal breathing or other signs of cardiac arrest
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 urgent medical assessment for a suspected stonefish sting, an unidentified fish sting with severe pain, multiple punctures, or a deep wound.
Urgent assessment is also appropriate when pain remains severe despite safe hot-water immersion, the sting is near a major joint, or sensation, movement, or blood flow appears reduced.
3
Does any of this apply?
Book an appointment
Arrange prompt, non-emergency medical care if redness or pain worsens after initially improving, pus develops, fever occurs, or part of a spine may still be in the wound.
You may also need a routine tetanus vaccination review.
The article also says
Contact a poison information service if one is available in your country.
It can provide advice based on the species, symptoms, and local treatment resources.
Every sign above is quoted from the sofidex article on Scorpion fish sting. If you are not sure, err upward — it is always reasonable to be checked.