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 for sudden trouble speaking or understanding speech, especially with facial drooping, arm weakness, vision loss, or severe dizziness.
These can be signs of a stroke.
Emergency care is also needed for serious breathing difficulty or choking that does not clear.
And for anyone, someone who:
A loss of skills that had already developed should be assessed promptly.
Seek prompt medical help after a head injury or when speech problems appear with new weakness, confusion, severe headache, balance problems, or swallowing difficulty.
Arrange a routine assessment if a child is not developing speech or language as expected, loses interest in communicating, is frequently difficult to understand, or struggles to understand everyday language.
The article also says
Adults should seek assessment for persistent changes in speech, language, voice, or communication.
A hoarse or altered voice that does not settle also needs medical review, especially if it is accompanied by pain, a neck lump, or difficulty swallowing.
Every sign above is quoted from the sofidex article on Speech and communication disorders. If you are not sure, err upward — it is always reasonable to be checked.