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 if a baby or child:
These concerns usually require a timely appointment rather than emergency care unless the child is acutely unwell.
Seek urgent medical assessment if a baby is feeding much less than usual, sweating or breathing rapidly during feeds, producing fewer wet diapers, becoming increasingly yellow, or developing unexplained bleeding or widespread purple spots.
These signs can have several causes but need prompt attention.
If you are pregnant and develop a rash or have close contact with someone who may have rubella, contact your maternity service or public health service promptly.
Call before attending in person so that staff can reduce the chance of exposing other pregnant people.
Keep routine follow-up appointments even when the child seems well.
Hearing loss, eye changes, thyroid problems, and developmental needs may not cause clear early symptoms.
The article also says
Contact the child's regular healthcare team soon if you notice a change in hearing, vision, balance, growth, feeding, or development.
Loss of a skill the child previously had also needs assessment.
Every sign above is quoted from the sofidex article on Congenital rubella. If you are not sure, err upward — it is always reasonable to be checked.