Before you do anything else, take a breath and look around. The first job in any emergency isn't to act - it's to understand what you're dealing with.
Not every urgent-feeling situation requires 911, and not every emergency announces itself clearly. As a general rule, treat it as an emergency if:
I need to know exactly what's wrong before I do anything. You don't need a diagnosis to call for help or keep someone safe.
If I'm not sure, I shouldn't get involved. Uncertainty is normal. Acting cautiously is still better than not acting at all.
Someone else has probably already called 911. In a crowd, this assumption is often wrong for everyone at once.
If you're genuinely unsure whether something is serious, treat it as if it is. Scaling back is always easier than catching up.