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 now if someone exposed to heat has confusion, unusual behaviour, a seizure, severe loss of coordination, collapse, or loss of consciousness.
Emergency assessment is also needed if the person has difficulty breathing, chest pain, repeated vomiting, signs of shock, or symptoms that are rapidly worsening.
Start cooling immediately while help is being arranged.
Seek urgent medical advice for suspected heat exhaustion that does not clearly improve after about 30 minutes of rest and active cooling.
Get advice sooner if the person is pregnant, medically vulnerable, unable to drink safely, or has very little urine.
Routine follow-up is appropriate after heatstroke, a hospital-treated heat illness, repeated episodes, or symptoms that continue after the heat exposure has ended.
This review can check recovery, medicines, organ function, and plans for returning to heat or exercise.
Every sign above is quoted from the sofidex article on Heat emergencies. If you are not sure, err upward — it is always reasonable to be checked.