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
Seek emergency help if you have foul-smelling stool along with:
Heavy rectal bleeding or black, tar-like stool with faintness
Severe or constant abdominal pain
Confusion, collapse, or difficulty staying awake
Signs of severe dehydration, such as very little urine and marked dizziness
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
Arrange same-day medical advice for blood in the stool, repeated vomiting, a high fever, worsening pain, or difficulty keeping fluids down.
You should also seek prompt advice for frequent watery diarrhea during or after antibiotics.
3
Does any of this apply?
Book an appointment
Book a routine appointment if the change lasts longer than about two weeks, keeps returning, or occurs with greasy or pale stool.
Other reasons to make an appointment include unexplained weight loss, ongoing bloating, persistent diarrhea, yellow skin or eyes, or a major lasting change in bowel habits.
The article also says
Children, older adults, and people with weakened immune systems can become dehydrated more quickly.
Seek advice earlier if fluid intake or urination is falling.
Every sign above is quoted from the sofidex article on Stools - foul smelling. If you are not sure, err upward — it is always reasonable to be checked.