[g.willikers]

When intervening in a situation that didn't directly involve you, is it always going to be clear as to who is the bad guy?

As far as deciding when imminent danger of bodily harm is about to happen, my guess would be after verbal commands are given, and making it clear that the police have been called and are on their way - even if it's untrue.
If the threat doesn't begin to fade at that point, and an attack continues, then it can probably be assumed the worst is about to happen.

Intervention can backfire, especially in places where self defense is not an accepted part of the legal system.
There's many parts of the world, and even this country, where the authorities are very jealous of their powers and might consider any interference as justification for causing someone a very bad time.