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 immediately if a baby:
Begin infant CPR and follow the emergency dispatcher's instructions.
Do not wait to see whether the baby improves.
Seek urgent medical assessment for a living baby who has pauses in breathing, marked breathing difficulty, a major colour change, unusual limpness, a seizure, or severe difficulty waking.
These events are not SIDS, but they may signal another serious problem.
Routine help is appropriate if you want advice about a baby's sleep space or have concerns about breathing during sleep.
After an infant death, arrange support if grief, anxiety, or sleep problems are making everyday life difficult.
The article also says
Get immediate crisis help if grief leads to thoughts of suicide, self-harm, or being unable to remain safe.
Every sign above is quoted from the sofidex article on Sudden infant death syndrome. If you are not sure, err upward — it is always reasonable to be checked.