Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Emergency care is appropriate for breathing difficulty, a seizure, sudden neurologic symptoms, or bleeding that remains heavy despite firm pressure.
Seek prompt medical advice for rapid growth, repeated bleeding, pain, warmth, swelling, drainage, an open sore, or a mark that begins affecting vision, feeding, hearing, or movement.
Routine follow-up depends on the type of birthmark.
A stable, harmless mark may only need review during regular health visits.
Birthmarks that are growing, being treated, or linked to possible complications may need more frequent appointments with dermatology, ophthalmology, plastic surgery, neurology, or a vascular-anomalies team.
Arrange a routine appointment if a birthmark gradually changes in color, texture, or thickness; if new spots appear; or if it causes distress or problems at school.
The article also says
Taking clear photographs with consistent lighting and a ruler for scale can help track changes.
Protecting the area from sunburn is also useful, particularly for pigmented birthmarks.
Sunscreen does not remove a birthmark, but it can reduce sun-related skin damage and color contrast.
Every sign above is quoted from the sofidex article on Birthmarks. If you are not sure, err upward — it is always reasonable to be checked.