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
Call emergency services for difficulty breathing, fainting, confusion, a major bleed, or signs of shock such as pale, cold or clammy skin with severe weakness.
These symptoms need immediate assessment.
And for anyone, someone who:
has had a seizure that will not stop, or does not wake up afterwards
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 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 prompt medical advice if you develop fever, headache, rash, or strong body aches while living in or after travelling to an area with dengue.
Mention where and when you travelled.
Testing for dengue and other infections may be needed.
If dengue has been diagnosed or is suspected, seek urgent same-day care for:
Severe or increasing abdominal pain
Repeated vomiting or inability to keep fluids down
Bleeding from the nose or gums
Blood in vomit, urine, or stool
Unusually heavy vaginal bleeding
Marked drowsiness, weakness, irritability, or restlessness
Much less urine than usual
Symptoms that worsen as the fever settles
And for anyone, someone who:
has a fever with a stiff neck, a rash that does not fade under pressure, or severe headache
The article also says
Children, pregnant people, older adults, and people with major heart, kidney, liver, blood, or immune conditions should seek medical advice early because closer monitoring may be appropriate.
Every sign above is quoted from the sofidex article on Dengue fever. If you are not sure, err upward — it is always reasonable to be checked.