Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
If you may harm yourself or someone else, or cannot keep yourself safe, contact emergency services or a local mental health crisis service immediately.
And for anyone, someone who:
Seek urgent medical help for chest pain, breathing difficulty, fainting, a seizure after leaving the facility, repeated seizures, new weakness on one side, or severe or worsening confusion.
The article also says
Contact the ECT team if headache, nausea, tiredness, or confusion is not improving; memory problems interfere with daily life; or you develop ongoing dental or muscle pain.
Every sign above is quoted from the sofidex article on Electroconvulsive therapy. If you are not sure, err upward — it is always reasonable to be checked.