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 for signs of severe dehydration or shock.
These include fainting, confusion, difficulty waking, very little or no urine, cold or mottled skin, rapid breathing, or severe weakness that prevents standing.
A seizure, breathing difficulty, or vomiting blood also needs emergency care.
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 advice if you or a child:
Cannot keep fluids down or is repeatedly vomiting
Has blood or black material in the stool
Has worsening or severe abdominal pain
Is passing much less urine than usual
Has persistent fever or appears increasingly unwell
Has diarrhoea after recent antibiotics or a hospital stay
Has symptoms that are not beginning to improve
Has a weakened immune system or a condition affected by fluid loss
The article also says
Babies, particularly those under 1 year, can become dehydrated quickly.
Seek advice early if a baby is feeding poorly, has fewer wet nappies, or is unusually sleepy.
Every sign above is quoted from the sofidex article on Gastroenteritis. If you are not sure, err upward — it is always reasonable to be checked.