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 for chest pressure, pain, or breathlessness that is new, severe, occurs at rest, or does not settle as expected.
Also seek emergency help if discomfort is accompanied by fainting, marked sweating, nausea, severe weakness, or pain spreading to the arm, jaw, back, or neck.
And for anyone, someone who:
A change in an established angina pattern can be urgent even if the discomfort eventually stops.
Seek prompt medical assessment if episodes are becoming more frequent, lasting longer, occurring with less activity, waking you from sleep, or responding less well to your usual treatment.
For symptoms that remain predictable but are limiting your activities, arrange a routine appointment.
Routine review is also appropriate for medicine side effects, uncertainty about exercise, or help managing cardiovascular risk factors.
The article also says
If you have an angina action plan, follow it.
Use prescribed fast-acting nitrate medicine as instructed while waiting for help.
Do not drive yourself to hospital.
Every sign above is quoted from the sofidex article on Stable angina. If you are not sure, err upward — it is always reasonable to be checked.