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 heat exposure is followed by confusion, fainting, a seizure, severe difficulty walking, or loss of consciousness.
These can be signs of heatstroke, which needs emergency treatment.
A person with heatstroke may have dry or sweaty skin, so do not use sweating alone to judge the risk.
Seek urgent same-day care if you cannot cool down or develop persistent dizziness, nausea, headache, weakness, cramps, or rapid breathing while hot.
Stop activity, move into shade or air conditioning, and sip cool water if you are fully alert and can swallow.
Get help if symptoms do not improve promptly.
A small, stable patch over an old scar is usually less urgent if the rest of your body sweats normally.
Book a routine appointment if the change is new, widespread, persistent, or repeatedly limits exercise.
You should also arrange an assessment if it began after starting a medicine or occurs with numbness, tingling, skin changes, unusual weakness, or problems controlling body temperature.
The article also says
While waiting for help, move to a cool place, remove unnecessary clothing, and cool the skin with wet cloths or cool water.
Do not give a drink to someone who is confused, unconscious, or unable to swallow safely.
Every sign above is quoted from the sofidex article on Absence of sweating. If you are not sure, err upward — it is always reasonable to be checked.