Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency help for signs that the body is not getting enough air or that the airway may be severely narrowed.
These include:
A child who is struggling to breathe should be kept as calm as possible while emergency help is arranged.
Do not force food, drink, or an examination of the throat.
Get prompt medical advice if breathing is faster or harder than usual, wheezing is new, a child has stridor at rest, or symptoms are steadily worsening.
Babies under 3 months with a fever should also be assessed promptly.
The article also says
Contact a healthcare professional if there are signs of dehydration, such as very little urine, a dry mouth, no tears when crying, or difficulty keeping fluids down.
Medical review is also appropriate for a persistent or returning fever, chest pain, worsening cough, or poor feeding.
People with a weakened immune system, a recent transplant, or a serious heart or lung condition should seek advice earlier in the illness, even when symptoms initially seem mild.
Every sign above is quoted from the sofidex article on Parainfluenza. If you are not sure, err upward — it is always reasonable to be checked.