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 for severe difficulty breathing, blue or grey lips or face, a seizure, inability to wake or stay awake, or severe chest pain.
For a child, emergency signs also include ribs pulling inward with each breath, severe dehydration, poor responsiveness, or a fever in a baby younger than 12 weeks.
Seek routine medical advice promptly if you develop flu symptoms and are pregnant, aged 65 or older, caring for a very young child with symptoms, or living with a condition that increases the risk of complications.
Early advice also matters if you have a weakened immune system.
Get urgent medical help for:
A child who cannot walk because of severe muscle pain also needs urgent assessment.
The article also says
Contact a clinician if symptoms are not starting to improve as expected, a fever persists or returns, you cannot drink enough, or a cough or underlying health condition is getting worse.
Every sign above is quoted from the sofidex article on H1N1 influenza. If you are not sure, err upward — it is always reasonable to be checked.