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
Get emergency help and begin a safe descent if you or someone with you has:
Breathlessness while resting
Confusion, unusual behaviour, or difficulty staying awake
Trouble walking straight, repeated stumbling, or inability to stand
A wet cough, chest tightness, or blue or grey lips
Rapid worsening after earlier mild altitude symptoms
Call local emergency or mountain rescue services when available.
Do not leave a confused, unsteady, or severely breathless person alone.
If immediate descent is unsafe, use oxygen or a portable pressure bag if available while arranging evacuation.
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
Does any of this apply?
Book an appointment
Arrange routine medical follow-up after HACE or HAPE, if symptoms continue after returning to a lower altitude, or before another high-altitude trip after a severe episode.
The article also says
Stop ascending if you develop a new headache with nausea, dizziness, unusual fatigue, or poor coordination after gaining altitude.
Rest at the same level and monitor symptoms closely.
Descend if they worsen or fail to improve.
Every sign above is quoted from the sofidex article on High altitude illness. If you are not sure, err upward — it is always reasonable to be checked.