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 your local emergency number if someone has trouble breathing, becomes unresponsive, has a prolonged seizure, has severe bleeding, or may have been poisoned or seriously injured.
Tell emergency staff exactly what you know, including any concern that symptoms may have been deliberately caused.
This may mean contacting emergency services, the police, or your local child or adult protection service.
Seek prompt safeguarding help if you believe a child or vulnerable adult is in immediate danger.
The article also says
Non-emergency concerns still deserve timely attention.
Contact a healthcare professional or safeguarding service if you notice repeated unexplained illnesses, major differences between reported and observed symptoms, possible interference with treatment, or unnecessary medical visits that seem to be harming someone.
You do not need to prove abuse before reporting a reasonable concern.
Record what you directly saw or heard rather than investigating on your own.
Avoid alerting or confronting the suspected person if doing so could increase danger.
Every sign above is quoted from the sofidex article on Munchausen syndrome by proxy. If you are not sure, err upward — it is always reasonable to be checked.