I don't think there's a clear bright line, any more than there's a clear bright line between being avoiding individuals you dislike and discrimination. That doesn't mean the distinction doesn't exist.

If one shopkeeper won't serve you, they're exercising their own rights and you can take your business elsewhere. But if every shopkeeper won't serve you, you're probably being discriminated against. I don't presume to draw the line precisely, but we recognise the difference.

I think we all know a mob when we see one.

If the mob is acting to achieve an ends I perceive as immoral, I am very quick to see a mob.

If the mob is acting to achieve an ends I perceive as righteous, I might never see the mob for what they really are.

I've found many people do even less well than I do, at this.

Clearly not, you'll need to be a bit more convincing than "I know it when I see it."

When confronted with a mob you won't have any questions or about whether is or is not.