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 a seizure, loss of consciousness, sudden severe weakness or another sudden neurological symptom.
These symptoms are not typical features of uncomplicated precocious puberty.
Seek prompt medical advice for unexplained vaginal bleeding, particularly when there are no other signs of puberty, or for new breast or genital changes associated with pain or a lump.
Seek urgent medical care if early puberty occurs with a severe or worsening headache, repeated vomiting, new vision changes, unusual thirst or urination, weakness, problems with balance, or a major change in behaviour.
Arrange a routine medical appointment if a girl develops breast tissue before age 8 or a boy has testicle or penis growth before age 9.
An appointment is also appropriate for rapid growth, fast progression of pubertal changes, or menstruation at an unusually young age.
It should still be discussed at a routine appointment, especially if it begins very early or progresses quickly.
The article also says
Pubic hair, acne or body odour without breast or testicle development is usually not an emergency.
Every sign above is quoted from the sofidex article on Precocious puberty. If you are not sure, err upward — it is always reasonable to be checked.