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 anyone is in immediate danger, the child has a serious injury, or you cannot keep the child or other people safe.
Emergency help is also needed for trouble breathing, blue or grey lips, a seizure, severe confusion, or difficulty waking.
Seek same-day medical advice if the child repeatedly injures themselves, makes credible threats to seriously hurt someone, or becomes dangerously aggressive.
The same applies if a sudden outburst follows a head injury or possible swallowing of medicine, alcohol, drugs, or chemicals.
Book a routine appointment if tantrums are frequent, prolonged, worsening, or disrupting family life, learning, friendships, or childcare.
Arrange an assessment if they continue well beyond the preschool years, occur without a clear trigger, or happen alongside concerns about speech, development, sleep, mood, or behaviour.
The article also says
You can also seek support if you feel unable to cope safely.
Asking for help does not mean that you have caused the behaviour.
Every sign above is quoted from the sofidex article on Temper tantrums. If you are not sure, err upward — it is always reasonable to be checked.